[Feature flag] Cleanup ff_limit_ssh_key_lifetime

Proposal

In %14.6 SSH key lifetime feature was released behind a default enabled feature flag. We need to remove the feature flag in the any of the milestones from %14.7

• Create a merge request to remove ff_limit_ssh_key_lifetime feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete ff_limit_ssh_key_lifetime --dev
  • /chatops run feature delete ff_limit_ssh_key_lifetime --staging
  • /chatops run feature delete ff_limit_ssh_key_lifetime
• Close this feature flag cleanup issue.

Relates to #346753 (closed)