Make the versions dropdown dynamic, don't hardcode them

We should really come up with something else than hardcoding the versions dropdown. Right now, we need to backport https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/content/_data/versions.yaml to all versions we want to include in the dropdown.

Ideally, this should be achieved with some javascript with data taken from a single place we'll be updating each month (maybe a json file at https://docs.gitlab.com/versions.json).