feat: Enhanced region support

[nightspotlight]

Description

• updated aws version constraint to >= 6.0
• added region input variable (see Enhanced Region Support)
• updated docs

Motivation and Context

This change will allow to simplify multi-region deployments of RDS resources. No longer need to maintain different provider aliases, one can simply pass region = "value" when calling the module. This will also allow for dynamic creation of multiple regional clusters with a for_each, like so:

locals {
  regions = toset(["us-east-1", "us-west-2", "eu-west-1"])
}

module "rds-aurora" {
  for_each = local.regions

  source = "terraform-aws-modules/rds-aurora/aws"

  region = each.value

  name = "rds-aurora-${each.value}"
}

Breaking Changes

Warning

AWS provider version 6.0.0 brings breaking changes, the list is extensive: https://github.com/hashicorp/terraform-provider-aws/releases/tag/v6.0.0

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
• I have executed pre-commit run -a on my pull request

[nightspotlight]

This is draft until I can apply it at work in the scheduled window in two weeks, unless somebody else can pick up and test. Plans cleanly though in both my work project and examples/postgresql.

[nightspotlight]

@antonbabenko , @bryantbiggs , ☝️😊

[bryantbiggs]

I think theres a few more changes we'll want to make since its a breaking change. its on the list and we'll get to it soon