Disable lookup of other ActiveSessions to determine admin mode status

What does this MR do?

Disable looking into other active user sessions to determine if admin mode is on / off. It limits the admin mode status to just the active browser session, compared to the previous approach of sharing the admin mode status between all active user sessions.

Other authentication methods are exempt from this:

• Sidekiq jobs have an injected metadata field at creation time that enables admin mode at execution time
• Sessionless auth methods (API, RSS) are auto-enabled for admin mode, in #42692 (closed) we'll enable PATs to be created with/without those rights.

Closes #35718 (closed)

🛠 with ❤ at Siemens

/cc @rpaik @bufferoverflow

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