Devise a way for feature flags to "expire"

As part of https://gitlab.com/gitlab-org/gitlab-ce/issues/49619, we anticipate adding more feature flags to the codebase. Because we don't want these flags to stagnate and become gitlab-ce~3011693, we want to ensure their timely removal is part of our "Definition of Done".

We want to find a way to have these flags essentially expire on a certain date, after which some action needs to be taken, such as opening an issue and/or pinging the relevant developers.

One idea is for this to be part of our release process, where upon tagging a release we find new feature flags and create issues for their removal X days from that point. I'm hopeful someone can suggest a better idea. 😃

cc @marin @smcgivern @mdelaossa @andr3 from our call about this this morning.

cc @yorickpeterse as this will likely be relevant to us as part of gitlab-com/www-gitlab-com!13471 (merged).