docs: (IAC-352) Subnet requirements need more clarity in doc

[dhoucgitter]

Changes

Update documentation to clarify what the AWS EKS subnet requirements are with and without BYO subnets.
Intentionally based off staging branch to pick up the latest doc changes there.

Tests

Ran the following tests to verify that added documentation was accurate.

Scenario	subnet_ids map	external db configured	rds multi_az value	tf apply Notes
1	1 private, 2 control plane (unique AZs)	yes	false	fails with Error 1
2	1 private, 2 control plane (unique AZs)	yes	true	fails with Error 1
3	1 private, 2 control plane (unique AZs), 2 database (unique AZs)	yes	true	successful
4	1 private, 2 control plane (unique AZs), 2 public (unique AZs)	yes	false	successful
5	1 private, 2 control plane (unique AZs)	no	false	successful

Error ¹

╷
│ Error: creating RDS DB Subnet Group (dh-619-default-pgsql-20231107223512640800000007): DBSubnetGroupDoesNotCoverEnoughAZs: The DB subnet group doesn't meet Availability Zone (AZ) coverage requirement. Current AZ coverage: us-west-1a. Add subnets to cover at least 2 AZs.
│ status code: 400, request id: e20f9fdf-ebde-4864-b457-d7a4ea6cc817
│
│ with module.postgresql["default"].module.db_subnet_group.aws_db_subnet_group.this[0],
│ on .terraform/modules/postgresql/modules/db_subnet_group/main.tf line 8, in resource "aws_db_subnet_group" "this":
│ 8: resource "aws_db_subnet_group" "this" {
│
╵

Reason for the failure to create RDS DB Subnet Group:

Even with the RDS module input parameter set to multi_az=false,  a DB subnet group has to have multiple subnets. It's a requirement of AWS RDS that you can't bypass.