docs: replace obsolete RFC 7231/7235 references with RFC 9110

[sriramakanthpendikatla]

Summary

Closes #2525

Updated obsolete RFC references in the documentation:

• RFC 7231 → RFC 9110 (HTTP Semantics)
• RFC 7235 → RFC 9110 (HTTP Semantics)

Updated links from tools.ietf.org to rfc-editor.org
as the canonical source.

Changes

• docs/user/tutorial.rst: Updated RFC 7231 link to RFC 9110
• docs/ext/rfc.py: Updated RFC 7231 section reference to RFC 9110

Testing

Documentation-only change. No code behaviour affected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6566f4a) to head (19d65c4).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2669   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7985      7985           
  Branches      1103      1103           
=========================================
  Hits          7985      7985           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.