Delete the `enable_environments_search_within_folder` feature flag

This feature flag was introduced in 15.7, which is more than 12 milestones ago.

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

Rollout issue: #382108 (closed)

The feature flag is enabled by default. Unless it's disabled on GitLab.com, you should keep the feature-flag code branch, otherwise, keep the other branch.

Remaining mentions of the feature flag (click to expand)

doc/ci/environments/index.md
53:> - [Generally available](https://gitlab.com/gitlab-org/gitlab/-/issues/382108) in GitLab 17.4. Feature flag `enable_environments_search_within_folder` removed.

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 groupenvironments to push those changes to this branch. If they are already removing this feature flag in another merge request then they can just close this merge request.

You can also see the status of the rollout by checking #382108 (closed) and https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=enable_environments_search_within_folder&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com.

This change was generated by gitlab-housekeeper 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.