Add missing IMAGE_TAG_EXT to referenced PostgreSQL image

What does this MR do and why?

We can see that attempting to build images for the stable branch 16-6-stable are failing due to missing this fix

https://gitlab.com/gitlab-org/build/CNG-mirror/-/jobs/5996387376

We need these images able to be built as part of the release-environments we are using to test deployments and QA of stable branches (e.g. https://gitlab.com/gitlab-org/gitlab/-/pipelines/1144819680)

This is just a backport of MR !1702 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

The process of backporting bug fixes into stable branches is tracked as part of an internal pilot. If you have questions about this process, please:

• Refer to the internal pilot issue for feedback or questions.
• Refer to the patch release runbook for engineers and maintainers for guidance.