Revert "Make the versions dropdown dynamic" from 15.2
We encountered a blocking bug with the new Versions menu when it's viewed in a version older than the main/pre-release site.
Since we're unlikely to have this fix merged before the 15.2 release, the menu should come out of this version, and we can give it another go in 15.3. It's OK to keep the new menu on the main branch since the bug was specific to older versions.
This MR reverts these:
How to test
- Visit the site in the review app
- The Versions menu look and function like it did in 15.1: https://docs.gitlab.com/15.1/