Backport of test(e2e): Update postgres version into release/0.15.x

[hc-github-team-secure-boundary]

Backport

This PR is auto-generated from #4701 to be assessed for backporting due to the inclusion of the label backport/0.15.x.

The below text is copied from the body of the original PR.

---

Some e2e tests were failing for the following reason

Error: creating RDS DB Instance (boundary-db-djcdlozc): InvalidParameterCombination: Cannot find version 15.3 for postgres
	status code: 400, request id: 97d6281e-b440-4885-b6fe-12a03827a6dc

  with module.create_boundary_cluster.aws_db_instance.boundary[0],
  on ../../modules/aws_boundary/rds.tf line 9, in resource "aws_db_instance" "boundary":
   9: resource "aws_db_instance" "boundary" {
  Validate: success!
  Plan: success!

This PR bumps the postgres version.