Add deprecation for disabling POSTGRES_ENABLED by default in Auto Deploy

For guidance on the overall deprecations, removals and breaking changes workflow, please visit Breaking changes, deprecations, and removing features

From GitLab 16.0, Auto DevOps deployments will no longer provision a PostgreSQL database by default.

Documentation updates are added in !108653 (merged).

Set the POSTGRES_ENABLED CI/CD variable to true to re-enable database provisioning.

Both self-managed users and SaaS users.

Labels

This issue is labeled deprecation, and with the relevant ~devops::, ~group::, and ~Category: labels.
This issue is labeled breaking change if the removal of the deprecated item will be a breaking change.

Timeline

Please add links to the relevant merge requests.

As soon as possible, but no later than the third milestone preceding the major release (for example, given the following release schedule: 14.8, 14.9, 14.10, 15.0 – 14.8 is the third milestone preceding the major release):
- A deprecation announcement entry has been created so the deprecation will appear in release posts and on the general deprecation page. - !108653 (merged)
- Documentation has been updated to mark the feature as deprecated. - !108653 (merged)
On or before the major milestone: A removal entry has been created so the removal will appear on the removals by milestones page and be announced in the release post. - !117972 (merged)
On the major milestone:
- The deprecated item has been removed. - !117980 (merged)
- If the removal of the deprecated item is a breaking change, the merge request is labeled breaking change.

Mentions

Your stage's stable counterparts have been @mentioned on this issue. For example, Customer Support, Customer Success (Technical Account Manager), Product Marketing Manager.
- To see who the stable counterparts are for a product team visit product categories
  - If there is no stable counterpart listed for Sales/CS please mention @timtams
  - If there is no stable counterpart listed for Support please mention @gitlab-com/support/managers
  - If there is no stable counterpart listed for Marketing please mention @cfoster3
Your GPM has been @mentioned so that they are aware of planned deprecations. The goal is to have reviews happen at least two releases before the final removal of the feature or introduction of a breaking change.