fix: update rds version

[timckt]

fix below error

https://concourse.cloud-platform.service.justice.gov.uk/teams/main/pipelines/environments-live/jobs/apply-live-b/builds/3335#L66be2b55:18392

FATA[1878] error running terraform on namespace formbuilder-platform-test-dev: unable to apply Terraform: exit status 1

Error: updating RDS DB Instance (cloud-platform-49237c541a078a82): operation error RDS: ModifyDBInstance, https response error StatusCode: 400, RequestID: 5f7c1bd4-73bb-44c2-9c6a-eb462f73b866, api error InvalidParameterCombination: Cannot upgrade postgres from 15.7 to 15.5

  with module.submitter-rds-instance-2.aws_db_instance.rds,
  on .terraform/modules/submitter-rds-instance-2/main.tf line 166, in resource "aws_db_instance" "rds":
 166: resource "aws_db_instance" "rds" {

https://concourse.cloud-platform.service.justice.gov.uk/teams/main/pipelines/environments-live/jobs/apply-live-e/builds/3340#L66be2b69:17068

FATA[2518] error running terraform on namespace offender-management-production: unable to apply Terraform: exit status 1

Error: updating RDS DB Instance (cloud-platform-0fe5d72ea3e106e5): operation error RDS: ModifyDBInstance, https response error StatusCode: 400, RequestID: f08fa376-98a4-4d4f-b890-2d99be6e491f, api error InvalidParameterCombination: Cannot upgrade postgres from 15.7 to 15.6

  with module.allocation-rds.aws_db_instance.rds,
  on .terraform/modules/allocation-rds/main.tf line 166, in resource "aws_db_instance" "rds":
 166: resource "aws_db_instance" "rds" {