(aws-rds): Improve Aurora Serverless Version Support With DatabaseClusterEngine (For ParameterGroup)

See #13936 for additional context.

The [CDK docs](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-rds.DatabaseClusterEngine.html) strongly recommend that you use a versioned engine when using non-serverless clusters, but are not as specific if you are using serverless clusters. So, I can create a ServerlessCluster using something like the following without issue:

```python
cluster = rds.ServerlessCluster(
self,
"Database",
engine=rds.DatabaseClusterEngine.AURORA_POSTGRESQL,
vpc=database_stack_props["vpc"],
backup_retention=cdk.Duration.days(7),
default_database_name="postgres",
deletion_protection=False,
enable_data_api=True,
parameter_group=rds.ParameterGroup.from_parameter_group_name(
self, "ParameterGroup", "default.aurora-postgresql10"
),
removal_policy=cdk.RemovalPolicy.SNAPSHOT,
storage_encryption_key=kms_key,
vpc_subnets=ec2.SubnetSelection(subnet_type=ec2.SubnetType.ISOLATED),
)
```

However, if you wish to create a parameter group for your serverless cluster and modify some of the default settings, you cannot use `from_parameter_group_name` since the imported resource is not able to be mutated (and therefore rds.force_ssl wont show up in your cdk synth output). So the following won't work:

```python
parameter_group = rds.ParameterGroup.from_parameter_group_name(
self, "ParameterGroup", "default.aurora-postgresql10"
)
parameter_group.add_parameter("rds.force_ssl", "1")
```

Additionally, you cannot use an unversioned engine as you will get an error: `jsii.errors.JSIIError: ParameterGroup cannot be used with an engine that doesn't specify a version`. So the following also won't work:
```python
parameter_group = rds.ParameterGroup(
self,
f"{construct_id}-ParameterGroup",
engine=rds.DatabaseClusterEngine.AURORA_POSTGRESQL,
parameters={
"rds.force_ssl": "1"
}
)
```

The **only** way I was able to create a parameter group for a serverless cluster is by using a specific named version:
```python
parameter_group = rds.ParameterGroup(
self,
"ParameterGroup",
engine=rds.DatabaseClusterEngine.aurora_postgres(
version=rds.AuroraPostgresEngineVersion.VER_10_14
),
parameters={
"rds.force_ssl": "1"
}
)
````

So, I am in a weird situation where my parameter group **MUST** be a named version, but my cluster doesn't. To keep things consistent, I should be using the same version for my ParameterGroup as my ServerlessCluster.

With Aurora Serverless v1, generally there is only a single version or two that is supported (per [AWS docs](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.relnotes.html)). For example, Aurora PostgreSQL Serverless v1 currently (as of the time of creating this issue) supports `10.12` only. Additionally, if AWS rolls out new version support, it upgrades any existing serverless clusters to that new version.

So, at the core of my problem:

1. If I don't set a specific ServerlessCluster version, I can't make a custom parameter group
2. If I set a specific ServerlessCluster version and Amazon bumps the supported version, my next cdk deploy could fail without any code changes

### Proposed Solution(s)

1. Create new default serverless parametergroups for Serverless (so instead of `default.aurora-postgresql10`, I can use `default.aurora-postgresql-serverless`. Note this may be something beyond CDK, or there may need to be some 🧙 magic 🧙 that maps `aurora-postgresql-serverless` to the latest supported parameter group version of Serverless (`default.aurora-postgresql10`)
2. Allow `rds.ParameterGroup.from_parameter_group_name` to be mutated, or create a new convenience method, such as `create_from_parameter_group_name`

Or...

1. Add more enums to [DatabaseClusterEngine](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-rds.DatabaseClusterEngine.html) or the underlying props classes like [AuroraPostgresEngineVersion](https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-rds.AuroraPostgresEngineVersion.html) that tracks/follows the version that AWS has for serverless (so if I create my database and the enum resolved to`10.12`, and they upgrade me from `10.12` to `10.14`, the enum then resolves to `10.14`).

Or...

Other ideas?

### Other

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

Start with DatabaseClusterEngine, AuroraPostgresEngineVersion, ServerlessCluster, and ParameterGroup.from_parameter_group_name in the CDK RDS implementation and API docs. Reproduce the serverless parameter-group examples, then determine which proposed behavior is compatible with AWS version support. Done means a documented, tested way to customize a serverless parameter group without pinning an unstable engine version.

Written by the indexing model from the issue text.