Add check for postgres nodes to postgres tasks

What does this MR do?

This change adds an extra safety check to prevent PostgreSQL database updates from running on the wrong servers. Previously, the system would attempt to update PostgreSQL on any server as long as the timing was right in the upgrade sequence. Now it also verifies that the server is actually designated as a PostgreSQL server before proceeding with the update. This prevents potential errors where database update tasks might accidentally run on servers that aren't meant to handle database operations, making the zero-downtime upgrade process more reliable and safer.

Related issues

Closes #1155 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gitlab-org/software-delivery/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 across all expected permutations.
  • Check that the changes work across upgrades.
  • Documentation created/updated in the same MR if applicable