GH Actions: Bump actions/checkout from 4 to 5

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/playground.yml

@@ -12,7 +12,7 @@ jobs:
       actions: read
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       # Prepare a folder named exactly like the repo as the plugin root.
       # If the repo already has such a folder (common for WP plugins), use it.