Add Postgres shutdown maintainence playbook

What does this MR do?

MR adds a maintenance playbook to ensure a graceful shutdown of Linux Postgres HA.

When doing a full shutdown of Linux Postgres HA failovers can be triggered by Patroni if the shutdown is not done cleanly, such as switching the VMs off via a Cloud Provider which can result in a random order. Depending on that order the primary may go down first and trigger the secondaries to start failover but are then subsequently shutdown also soon after, leading to failover itself failing and leaving the nodes in a precarious state.

Playbook ensure Postgres is shutdown secondaries first and then primary second to avoid if required.

Related issues

Relates omnibus-gitlab#5969

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.