SSH: Do not allow expiration enforcement to be disabled

Problem to solve

Since #250480 (closed), GitLab allows administrators to disable the enforcement of SSH key expiration. That could cause unexpected behavior for users when the setting is enabled. Specifically, users might not be aware when their credentials (or others') are expired but still in use.

Proposal

We should deprecate and remove the ability for administrators to disable the enforcement of SSH key expiration:

• Deprecation announcement and documentation update in %14.8. See !79905 (merged)
• Feature removal in %15.0
  • Admin setting for enforce_ssh_key_expiration
  • Admin documentation

Note: This is a revert of the work done under #250480 (closed).