Non-empty plans when migrating existing optional+computed nested Set blocks to Terraform Plugin Framework

### Module version

```
1.4.2
```
We are currently migrating existing Plugin-SDK based resources to Plugin Framework (Plugin protocol v6). In below resource, we have some **optional+computed** set block attributes where certain nested attributes _if not configured/partially configured_ by the user in the config, the API/provider will still return those attributes. That response is currently persisted in the state.

In order to migrate these blocks to the Plugin Framework, we have tried using schema.SetNestedBlock but the returned plan after upgrade to the Framework-migrated resource is always non-empty. **This will be a breaking change for our users.**

**What is the recommended non-breaking way to migrate collection attributes such as these?**

### Relevant provider source code

Terraform Plugin SDK based schema for "replication_specs" block:
```go
"replication_specs": {
Type: schema.TypeSet,
Optional: true,
Computed: true,
Elem: &schema.Resource{
Schema: map[string]*schema.Schema{
"id": {
Type: schema.TypeString,
Optional: true,
Computed: true,
},
"num_shards": {
Type: schema.TypeInt,
Required: true,
},
"regions_config": {
Type: schema.TypeSet,
Optional: true,
Computed: true,
Elem: &schema.Resource{
Schema: map[string]*schema.Schema{
"region_name": {
Type: schema.TypeString,
Required: true,
},
"electable_nodes": {
Type: schema.TypeInt,
Optional: true,
Computed: true,
},
"priority": {
Type: schema.TypeInt,
Optional: true,
Computed: true,
},
"read_only_nodes": {
Type: schema.TypeInt,
Optional: true,
Default: 0,
},
"analytics_nodes": {
Type: schema.TypeInt,
Optional: true,
Default: 0,
},
},
},
},
"zone_name": {
Type: schema.TypeString,
Optional: true,
Default: "ZoneName managed by Terraform",
},
},
},
Set: func(v any) int {
var buf bytes.Buffer
m := v.(map[string]any)
buf.WriteString(fmt.Sprintf("%d", m["num_shards"].(int)))
buf.WriteString(m["zone_name"].(string))
buf.WriteString(fmt.Sprintf("%+v", m["regions_config"].(*schema.Set)))
return advancedcluster.HashCodeString(buf.String())
},
},
```

Terraform Plugin Framework migrated schema:
```go
func clusterRSReplicationSpecsSchemaBlock() schema.SetNestedBlock {
return schema.SetNestedBlock{
NestedObject: schema.NestedBlockObject{
Attributes: map[string]schema.Attribute{
"id": schema.StringAttribute{
Optional: true,
Computed: true,
},
"num_shards": schema.Int64Attribute{
Required: true,
},
"zone_name": schema.StringAttribute{
Optional: true,
Computed: true,
Default: stringdefault.StaticString("ZoneName managed by Terraform"),
},
},
Blocks: map[string]schema.Block{
"regions_config": schema.SetNestedBlock{
NestedObject: schema.NestedBlockObject{
Attributes: map[string]schema.Attribute{
"analytics_nodes": schema.Int64Attribute{
Optional: true,
Computed: true,
Default: int64default.StaticInt64(0),
},
"electable_nodes": schema.Int64Attribute{
Optional: true,
Computed: true,
},
"priority": schema.Int64Attribute{
Optional: true,
Computed: true,
},
"read_only_nodes": schema.Int64Attribute{
Optional: true,
Computed: true,
Default: int64default.StaticInt64(0),
},
"region_name": schema.StringAttribute{
Required: true,
},
},
},
},
},
},
}
}
```

### Terraform Configuration Files

Use-case 1 (no blocks configured):
```hcl
main.tf:
resource "mongodbatlas_cluster" "cluster-no-blocks" {
project_id = mongodbatlas_project.project-tf.id
provider_name = "AWS"
name = "tfCluster1"
backing_provider_name = "AWS"
provider_region_name = "US_EAST_1"
provider_instance_size_name = "M10"
auto_scaling_compute_enabled = false
auto_scaling_compute_scale_down_enabled = false
provider_auto_scaling_compute_min_instance_size = "M10"
provider_auto_scaling_compute_max_instance_size = "M20"
}

terraform.tfstate (including only concerned blocks here due to large resource config):
"replication_specs": [
{
"id": "....",
"num_shards": 1,
"regions_config": [
{
"analytics_nodes": 0,
"electable_nodes": 3,
"priority": 7,
"read_only_nodes": 0,
"region_name": "US_EAST_1"
}
],
"zone_name": "Zone 1"
}
],
```

Use-case 2 (all blocks configured):
```hcl
main.tf:
resource "mongodbatlas_cluster" "cluster-multi-region-all-blocks" {
project_id = mongodbatlas_project.project-tf.id
name = "cluster-test-multi-region"
num_shards = 1
cloud_backup = true
cluster_type = "REPLICASET"

provider_name = "AWS"
provider_instance_size_name = "M10"

advanced_configuration { # block can be partially configured by user
minimum_enabled_tls_protocol = "TLS1_2"
default_read_concern = "available"
}

bi_connector_config {
enabled = false
read_preference = "secondary"
}

replication_specs {
num_shards = 1
regions_config {
region_name = "US_EAST_1"
electable_nodes = 3
priority = 7
read_only_nodes = 0
}
regions_config {
region_name = "US_EAST_2"
electable_nodes = 2
priority = 6
read_only_nodes = 0
}
regions_config {
region_name = "US_WEST_1"
electable_nodes = 2
priority = 5
read_only_nodes = 2
}
}
}

terraform.tfstate (including only concerned blocks here due to large resource config):
"replication_specs": [
{
"id": "....",
"num_shards": 1,
"regions_config": [
{
"analytics_nodes": 0,
"electable_nodes": 2,
"priority": 5,
"read_only_nodes": 2,
"region_name": "US_WEST_1"
},
{
"analytics_nodes": 0,
"electable_nodes": 2,
"priority": 6,
"read_only_nodes": 0,
"region_name": "US_EAST_2"
},
{
"analytics_nodes": 0,
"electable_nodes": 3,
"priority": 7,
"read_only_nodes": 0,
"region_name": "US_EAST_1"
}
],
"zone_name": "ZoneName managed by Terraform"
}
],
```

### Debug Output

### Expected Behavior

After user upgrades to new provider version with the framework-migrated resource, user should not see any planned changes for optional+computed lists/set blocks when running `terraform plan` and not receive errors when running `terraform apply`.

### Actual Behavior

On running `terraform plan` below plan was produced by Terraform:
**Use-case 1 (no blocks configured):**
```
~ update in-place

Terraform will perform the following actions:

# mongodbatlas_cluster.cluster-no-blocks will be updated in-place
~ resource "mongodbatlas_cluster" "cluster-no-blocks" {
id = "Y2x1c3Rlcl9pZA==:NjU2ODhiMmYzZDI3MWYzZjg2MGI0NjQw-Y2x1c3Rlcl9uYW1l:dGZDbHVzdGVyMQ==-cHJvamVjdF9pZA==:NjRlY2MxNTkyNzVmMjM1OWY0Y2FlODEw-cHJvdmlkZXJfbmFtZQ==:QVdT"
name = "tfCluster1"
~ num_shards = 1 -> (known after apply)
# (34 unchanged attributes hidden)

- replication_specs {
- id = "......." -> null
- num_shards = 1 -> null
- zone_name = "Zone 1" -> null

- regions_config {
- analytics_nodes = 0 -> null
- electable_nodes = 3 -> null
- priority = 7 -> null
- read_only_nodes = 0 -> null
- region_name = "US_EAST_1" -> null
}
}
}
```

**Use-case 2 (all blocks configured):**
```
~ update in-place

Terraform will perform the following actions:

# mongodbatlas_cluster.cluster-multi-region-all-blocks will be updated in-place
~ resource "mongodbatlas_cluster" "cluster-multi-region-all-blocks" {
id = "Y2x1c3Rlcl9pZA==:NjU2ODhlZTgzZDI3MWYzZjg2MGI0ZTY3-Y2x1c3Rlcl9uYW1l:dGYtbXVsdGktcmVnaW9uLWFsbC1ibG9ja3M=-cHJvamVjdF9pZA==:NjRlY2MxNTkyNzVmMjM1OWY0Y2FlODEw-cHJvdmlkZXJfbmFtZQ==:QVdT"
name = "tf-multi-region-all-blocks"
# (32 unchanged attributes hidden)

- replication_specs {
- id = "......." -> null
- num_shards = 1 -> null
- zone_name = "ZoneName managed by Terraform" -> null

- regions_config {
- analytics_nodes = 0 -> null
- electable_nodes = 2 -> null
- priority = 5 -> null
- read_only_nodes = 2 -> null
- region_name = "US_WEST_1" -> null
}
- regions_config {
- analytics_nodes = 0 -> null
- electable_nodes = 2 -> null
- priority = 6 -> null
- read_only_nodes = 0 -> null
- region_name = "US_EAST_2" -> null
}
- regions_config {
- analytics_nodes = 0 -> null
- electable_nodes = 3 -> null
- priority = 7 -> null
- read_only_nodes = 0 -> null
- region_name = "US_EAST_1" -> null
}
}
+ replication_specs {
+ num_shards = 1
+ zone_name = "ZoneName managed by Terraform"

+ regions_config {
+ analytics_nodes = 0
+ electable_nodes = 2
+ priority = 5
+ read_only_nodes = 2
+ region_name = "US_WEST_1"
}
+ regions_config {
+ analytics_nodes = 0
+ electable_nodes = 2
+ priority = 6
+ read_only_nodes = 0
+ region_name = "US_EAST_2"
}
+ regions_config {
+ analytics_nodes = 0
+ electable_nodes = 3
+ priority = 7
+ read_only_nodes = 0
+ region_name = "US_EAST_1"
}
}
```

### Steps to Reproduce

1. run `terraform init` for provider v.X (contains-Plugin-SDK-based-resource-implementation) for above resource
2. run `terraform plan`
4. run `terraform apply`
5. run `terraform plan` again -> plan returns `No changes.`
6. update provider version to v.Y (contains-Plugin-Framework-based-resource-implementation) for above resource, run `terraform init --upgrade`
7. run `terraform plan` -> Plan is not empty

### References

- Related issue for list blocks migration - https://github.com/hashicorp/terraform-plugin-framework/issues/883
- replication_specs [Plugin-SDK schema](https://github.com/mongodb/terraform-provider-mongodbatlas/blob/master/internal/service/cluster/resource_cluster.go#L215) (note Set [function](https://github.com/mongodb/terraform-provider-mongodbatlas/blob/master/internal/service/cluster/resource_cluster.go#L270))