Upgrade flipper for Rails 7 support

What does this MR do and why?

Upgrades flipper to a version that supports Rails 7

• https://my.diffend.io/gems/flipper/0.21.0/0.25.0

  A lot of the changes are related to the UI / API which we don't use: https://github.com/jnunemaker/flipper/blob/master/Changelog.md. There is a new FailSafe adapter that's something we might be able to use. Right now, we have the #feature_flags_available? method to rescue certain exceptions. This could make the code a bit cleaner. We can do this separately in another MR.

• https://my.diffend.io/gems/flipper-active_record/0.24.1/0.25.0

  Adds a base class to Flipper models so that it's easier to use with multiple databases: https://github.com/jnunemaker/flipper/pull/629. We aren't affected because we pass in a custom model class: https://gitlab.com/gitlab-org/gitlab/-/blob/c94aa949f6a9c9d90dcad67c0adea819d7792c71/lib/feature.rb#L248

• https://my.diffend.io/gems/flipper-active_support_cache_store/0.21.0/0.25.0

  Relaxes ActiveSupport dependency and removes unneeded requires

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.