[rustatian]

Reason for This PR

HTTP/3 was the last feature behind the -e (--enable-experimental) flag. It is stable and starts from the http3 section alone.

Description of Changes

• The plugin starts the HTTP/3 listener when the http3 section is present. The experimental flag no longer gates it.
• The api.Configurer interface drops the Experimental method, and the plugin no longer stores the flag.
• The schema description for http3 drops the experimental note.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.47%. Comparing base (efaf69e) to head (c67e222).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   88.48%   88.47%   -0.01%     
==========================================
  Files          29       29              
  Lines        1346     1345       -1     
==========================================
- Hits         1191     1190       -1     
  Misses        155      155              

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