Delete the elastic_migration_worker feature flag

This feature flag was introduced in 14.8, 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: #352424 (closed)

Remaining mentions of the feature flag (click to expand)

config/initializers/1_settings.rb
846:  Settings.cron_jobs['elastic_migration_worker'] ||= {}
847:  Settings.cron_jobs['elastic_migration_worker']['cron'] ||= '*/5 * * * *'
848:  Settings.cron_jobs['elastic_migration_worker']['job_class'] ||= 'Elastic::MigrationWorker'

doc/development/advanced_search/tips.md
143:>- [Generally available](https://gitlab.com/gitlab-org/gitlab/-/issues/352424) in GitLab 18.1. Feature flag `elastic_migration_worker` removed.

doc/integration/advanced_search/elasticsearch.md
804:With the `elastic_migration_worker_enabled` application setting,

ee/app/helpers/ee/application_settings_helper.rb
98:        :elastic_migration_worker_enabled

ee/app/models/ee/application_setting.rb
31:        elastic_migration_worker_enabled: [:boolean, { default: true }]

ee/app/validators/json_schemas/application_setting_ee_search.json
47:    "elastic_migration_worker_enabled": {

ee/spec/helpers/ee/application_settings_helper_spec.rb
29:        elastic_migration_worker_enabled

ee/spec/models/application_setting_spec.rb
61:    it { expect(setting.elastic_migration_worker_enabled).to be(true) }

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 groupglobal search 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.

TODO for the reviewers before merging this MR

• See the status of the rollout by checking #352424 (closed), https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=elastic_migration_worker&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 elastic_migration_worker.
• Search for references to ElasticMigrationWorker in frontend part of code
• Search for references to elastic_migration_worker in code
• Check if we need to remove any Gem or other related code by looking at the changes in !80310 (merged)

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.