Update rack-oauth2 1.2.1 -> 1.9.3

What does this MR do?

Update rack-oauth2 to latest upstream release.

Are there points in the code the reviewer needs to double check?

Only ee_compat_check failed.

Why was this MR needed?

Debian already has 1.4.0, but updating to latest is best option.

Screenshots (if relevant)

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
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #21732 (closed)