Migrate CE & EE controller specs to consider admin mode

What does this MR do?

Migration of specs to remove auto admin mode mocking for controllers:

• Keep mocking for Admin::** controllers, since they are protected by the EnforcesAdminAuthentication concern. Exception for Admin::SessionsController as it is the entry point for admin mode
• A subset of normal controllers get tested for admin mode off / on contexts

🛠 with ❤ at Siemens

/cc @bufferoverflow @rpaik @ifarkas @reprazent

Related to #31511 (closed)

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