Default to PostgreSQL 16 for fresh installs

What does this MR do?

Default to PostgreSQL 16 for fresh installs

• Fresh installs now default to PostrgreSQL 16.
• The default can be overridden by setting postgresql['version']=14.
• Existing installations do not upgrade automatically.

Related issues

Closes https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8829+

Last time we did this: Default to PostgreSQL 14 for fresh installations (!7294 - merged)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated. gitlab!182553 (merged)
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Test Plan

1. Fresh install
   • Setup a fresh install
   • Confirm the PostgreSQL version is 16 postgres --version
   • Setup another fresh install with postgresql['version'] = 14
   • Confirm the PostgreSQL version is 14
2. Upgrades
   • Setup a fresh 17.9 install
   • Confirm the default PostgreSQL version is 14
   • Upgrade the node to this branch
   • Confirm the PostgreSQL is not upgraded to 14
   • run gitlab-ctl pg-upgrade -V 16
   • Confirm the upgrade passes
   • Downgrade to PostgreSQL 14 gitlab-ctl revert-pg-upgrade 14