[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 #352008 (comment 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 feature flag.