Step-up auth: Mark active session as step-up authenticated

Problem/Motivation

GitLab's active sessions page shows when a session has admin mode enabled, but it does not indicate when a session is step-up authenticated. This creates a gap in transparency and security management.

Without clear visibility into which sessions have completed step-up authentication, users cannot:

1. Identify sessions with elevated security privileges
2. Understand the security state of their login sessions
3. Make informed decisions about session management

Proposal

Enhance the active sessions page to indicate when a session has been step-up authenticated.

Consider the following aspects:

• Adding a step-up authentication status indicator to active sessions
• Clearing the step-up authentication state when admin mode is disabled