Revert "Merge branch '667-versions-menu-1312' into '13.12'"

Unfortunately, the solution we need to revert creates the following menu items and links:

• 15.2: https://docs.gitlab.com/13.12/
• 15.1: https://docs.gitlab.com/15.1/13.12/
• 15.0: https://docs.gitlab.com/15.0/13.12/
• 14.10: https://docs.gitlab.com/14.10/13.12/
• 13.12: https://docs.gitlab.com/13.12/13.12/

In the case of the menu for 13.12, we need 15.2 to always point to https://docs.gitlab.com/, and to not append the version to all the other links.

CC @sarahgerman