[Feature flag] Enable Runner token expiration enforcement by default for self-managed GitLab and GitLab.com
## Overview The Runner token expiration enforcement feature for GitLab administrators was released in 15.3 behind a feature flag. Based on the reasoning in https://gitlab.com/gitlab-org/gitlab/-/issues/352008#note_1125566511, we should now consider enabling this feature as a default option on self-managed GitLab. Even with the feature flag removed, the functionality is disabled by default and needs configuration of token expiry period in order to take effect. ## Proposal * Remove the [enforce_runner_token_expires_at](https://gitlab.com/gitlab-org/gitlab/-/issues/352008) feature flag.
issue