Added local_store to gitlab.yml file

What does this MR do?

Relates to #301159 (closed)

This MR introduces pages: local_store to config/gitlab.yml. Following this introduction, this new config is used along pages_update_legacy_storage FF occurrences, as we'll be transitioning from this FF to Settings.pages.local_store.enabled. Related rspecs were updated accordingly.

Other than that:

• this MR introduces a new initializer, config/initializers/pages_storage_check.rb - it makes sure that at least one enabled attribute (for pages: local_store or pages: object_store) is set to true.
• Omnibus MR: omnibus-gitlab!5058 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #301159 (closed)