chore: update checkout action version to v3: (#4598)

* Update the version of the checkout action (for GitHub Actions) in
  `clang-format.yml` and `levelization.yml`.
  * The previous version, v2, was raising deprecation warnings due to
    its reliance on Node.js 12.
  * The latest checkout action version, v3, uses Node.js 16.

.github/workflows/levelization.yml

@@ -8,7 +8,7 @@ jobs:
     env:
       CLANG_VERSION: 10
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Check levelization
       run: Builds/levelization/levelization.sh
     - name: Check for differences