Delete the reject_unsigned_commits_by_gitlab feature flag

This feature flag was introduced in 13.11, which is more than 4 milestones ago.

As part of our process we want to ensure feature flags don't stay too long in the codebase.

Rollout issue: #326775

Remaining mentions of the feature flag (click to expand)

doc-locale/ja-jp/user/project/repository/push_rules.md
292:このプッシュルールを使用してプロジェクトでWeb IDEを介してコミットできるようにするには、GitLab管理者は`reject_unsigned_commits_by_gitlab`機能フラグを[フラグを使用して](../../../administration/feature_flags.md)無効にする必要があります。
295:Feature.disable(:reject_unsigned_commits_by_gitlab)

doc/user/project/repository/push_rules.md
353:> - [Generally available](https://gitlab.com/gitlab-org/gitlab/-/issues/326775) in GitLab 18.4. Feature flag `reject_unsigned_commits_by_gitlab` removed.

Currently the feature flag is enabled on production

It is possible that this MR will still need some changes to remove references to the feature flag in the code. At the moment the gitlab-housekeeper is not always capable of removing all references so you must check the diff and pipeline failures to confirm if there are any issues. It is the responsibility of groupide to push those changes to this branch.

Note: If you do not want to remove this feature flag at this time, you can add an intended_to_rollout_by_date attribute in the feature flag YAML file to prevent automated removal.

TODO for the reviewers before merging this MR

• See the status of the rollout by checking #326775, https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=reject_unsigned_commits_by_gitlab&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com
• Verify the feature flag status via chatops by running /chatops run feature get reject_unsigned_commits_by_gitlab.
• Search for references to RejectUnsignedCommitsByGitlab in frontend part of code
• Search for references to reject_unsigned_commits_by_gitlab in code
• Check if we need to remove any Gem or other related code by looking at the changes in !58453 (merged)

This change was generated by gitlab-housekeeper in CI using the Keeps::DeleteOldFeatureFlags keep.

To provide feedback on your experience with gitlab-housekeeper please create an issue with the label GitLab Housekeeper and consider pinging the author of this keep.