[Auth gem] Deprecate OmniAuth Facebook

Context

• Last gem release was in 2021 and we are 5 major versions behind (running 4.0.0, which was released in 2016)
• Gem is looking for maintainers: https://github.com/simi/omniauth-facebook/blob/master/README.md#omniauth-facebook--
• Very little usage by GitLab customers
• We do not use Facebook OAuth on Gitlab.com

Definition of done

• Deprecation announcement - %16.2 or %16.3 | MR
• Removal of gem and all references as a breaking change - %17.0
• Removal of all Facebook identity records via migration (Identity.where(provider: 'facebook')) in the database - %17.1