feat: switch YAML extension preference from .yml to .yaml

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Switch from .yml files to .yaml #2309
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Renames all .yml files to .yaml, and switches the yml/file-extension lint rule to its default of .yaml.

Also updates intakeFileAsYaml to check both .yaml and .yml, automatically.

🎁

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (d7c0947) to head (a5ab95d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2310   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files         138      138           
  Lines         933      933           
  Branches      242      243    +1     
=======================================
  Hits          919      919           
  Misses          9        9           
  Partials        5        5           

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[github-actions]

🎉 This is included in version v2.57.0 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀