fix: pin all GitHub Actions to full commit SHAs (resolves SonarCloud S7637) (#90)

Pin all floating version tags (@v1, @v2, @v3, @v4, @master) to full commit
SHAs across all workflow files to eliminate supply chain security risk.

Actions pinned:
- actions/checkout@v3 -> f43a0e5 (v3.6.0) in release.yml, sync-wiki.yml
- actions/checkout@v4 -> 34e1148 (v4) in tests.yml
- shivammathur/setup-php@v2 -> accd612 (v2) in all workflows
- actions/setup-node@v4 -> 49933ea (v4) in playground-tests*.yml, wordpress-tests.yml
- actions/upload-artifact@v4 -> ea165f8 (v4) in playground-tests*.yml, wordpress-tests.yml
- softprops/action-gh-release@v1 -> de2c0eb (v1) in release.yml
- codacy/codacy-analysis-cli-action@v4 -> 562ee3e (v4) in code-quality.yml
- github/codeql-action/upload-sarif@v3 -> 603b797 (v3) in code-quality.yml
- swissspidy/wp-performance-action@v2.0.3 -> b7e3ffc (v2.0.3) in playground-tests.yml
- SonarSource/sonarqube-scan-action@master -> 9598b8a in sonarcloud.yml

Closes #89

.github/workflows/sonarcloud.yml

@@ -49,7 +49,7 @@ jobs:
 
       - name: SonarCloud Scan
         if: steps.check_token.outputs.skip != 'true'
-        uses: SonarSource/sonarqube-scan-action@master
+        uses: SonarSource/sonarqube-scan-action@9598b8a83feef37de07f549027fab50ecffe6a6e # master
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           SONAR_TOKEN: ${{ secrets.SONARCLOUD_GITHUB }}