Block git push over HTTP when database is in read-only mode

What does this MR do?

This MR blocks git pushes over HTTP when db is read-only, which happens on Geo secondaries.

This is part of the discussion !46045 (diffs, comment 441993716) on blocking non-get HTTP requests in the under-development maintenance mode. The main assertion is:

pushes (git_receive_pack) were only intended to be allowed on Geo secondaries. It's fine because they proxy pushes to the primary

(But pushes were not intended to be allowed on non-Geo secondaries when the DB is read-only)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.