Incorrect config validation - `If you have set either region_kms_key_ids or kms_key_id, encrypt_boot must also be true.`

#### Overview of the Issue / Reproduction steps

Incorrect config validation prevents use of the most efficent build behaviour.

Suppose you want to:
* Create an encrypted AMI...
* ... and do so most efficiently, by using `launch_block_device_mappings` to launch the build instance encrypted initially, rather than performing a copy/encrypt operation to apply encryption after building an unencrypted AMI first...
* ... but you also want the final AMI to be copied to other regions using specified keys

The plugin rejects such a configuration with the message:
```
If you have set either region_kms_key_ids or kms_key_id, encrypt_boot must also be true.
```

I don't think this is correct. The technique of encrypting via `launch_block_device_mappings` is documented at https://developer.hashicorp.com/packer/integrations/hashicorp/amazon/latest/components/builder/ebs within the documentation for `kms_key_id` :

> If you have a custom kms key you'd like to apply to the launch volume, and are only building in one region, it is more efficient to leave this and encrypt_boot empty and to instead set the key id in the launch_block_device_mappings (you can find an example below). This saves potentially many minutes at the end of the build by preventing Packer from having to copy and re-encrypt the image at the end of the build.

Just because I've specified `region_kms_key_ids` to customize the keys to be used for the copy to `ami_regions`, shouldn't prevent me from using this build technique.

### Simplified Packer Buildfile

```
packer {
required_plugins {
amazon = {
version = "~> 1.0"
source = "github.com/hashicorp/amazon"
}
}
}

variable "build_region" {
type = string
default = "eu-west-2"
}

variable "distribution_regions" {
type = list(string)
default = [
"eu-west-1",
]
}

variable "kms_key_id" {
type = string
default = "mrk-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
}

source "amazon-ebs" "debian" {
ami_name = "my-ami-{{timestamp}}"
region = var.build_region
kms_key_id = var.kms_key_id
ami_regions = var.distribution_regions
region_kms_key_ids = { for region in var.distribution_regions : region => var.kms_key_id }
instance_type = "t3.2xlarge"
launch_block_device_mappings {
device_name = "/dev/xvda"
volume_size = 8
volume_type = "gp3"
delete_on_termination = true
}
source_ami_filter {
filters = {
name = "debian-12-amd64-*"
root-device-type = "ebs"
virtualization-type = "hvm"
architecture = "x86_64"
}
most_recent = true
owners = ["136693071363"] # https://wiki.debian.org/Cloud/AmazonEC2Image
}
ssh_username = "admin"
}

build {
sources = ["source.amazon-ebs.debian"]
}
```