Change Sign Out route from a DELETE to a GET

What does this MR do?

Changes the HTTP method for the destroy_user_session action (the Sign Out link) from DELETE to GET, facilitating simpler single-sign-out integration with external apps/sites.

Why was this MR needed?

Solves #39708 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together