[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.
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