Upgrade GitHub Actions to the latest version (v6)

Thanks to salmanmkc@ who suggested this in #571.

PiperOrigin-RevId: 854113440
Change-Id: Ib71810028c7492604978eb1c2f9e46d5f57486b5

Author: Augustin-Zidek

.github/workflows/ci.yaml

@@ -21,9 +21,9 @@ jobs:
             python-version: '3.11'
             os: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies