ci: consolidate checks into single workflow with release-please gate

[josephfusco]

Consolidates PHPUnit, Multisite, PHPCS, PHPStan, and Plugin Check into a single ci.yml with a gate job that skips all checks on release-please branches and PRs (refs #119).

Note for reviewers: Required status check names will change from e.g. PHPUnit / PHP 7.4 to CI / PHP 7.4. If branch protection has the old names configured, those rules will need updating by an org admin after merge.

[github-advanced-security]

zizmor found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.24%. Comparing base (a2231d0) to head (bc0f7ee).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #124   +/-   ##
=========================================
  Coverage     27.24%   27.24%           
  Complexity      174      174           
=========================================
  Files            15       15           
  Lines          1817     1817           
=========================================
  Hits            495      495           
  Misses         1322     1322           

Flag	Coverage Δ
phpunit	27.24% <ø> (ø)

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[github-actions]

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