Removed Postgres 13 from RELATED_IMAGES.

[ValClarkson]

Now that we've had 2 patch releases of Postgres 15 we are dropping postgres 13.

Issue [sc-17907]

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• New feature
• Bug fix
• Documentation
• Testing enhancement
• Other

What is the current behavior (link to any open issues here)?

What is the new behavior (if this is a feature change)?

• Breaking change (fix or feature that would cause existing functionality to change)

Other Information:

[mloskot]

Since it says "dropping support" and given the notification displayed below,
does this mean PostgreSQL 13 (and older) should be removed from the "Postgres" list for PGO 5.3.0 here too?
https://github.com/CrunchyData/postgres-operator/blob/fd5c247fff38b15e134caf3cd40901cfbb927215/docs/content/references/components.md