Conditional mocking of admin mode in specs by directory

What does this MR do?

Prepare to break !27405 (closed) in chunks by mocking admin mode in spec by directory.

• Keeps the :do_not_mock_admin_mode metadata and introduces a new :enable_admin_mode. The mocking of admin mode is still active like previously, but limited to a set of directories
• We'll break !27405 (closed) in multiple MRs that each tackle a subset of the directories, apply the changes in there to the specs and remove mocking of the relevant directories
• This approach also has the nice property of forcing new specs added outside of these directories to have to handle admin mode, as will be the case for all of them when the migration is finished

Related to #31511 (closed)

🛠 with ❤ at Siemens

/cc @bufferoverflow @rpaik @reprazent @ifarkas @rymai

Screenshots

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