[ci] Update action/checkout to v4

Author: aschuh-hf

.github/workflows/docs.yml

@@ -28,7 +28,7 @@ jobs:
       id-token: write
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
       # See https://dev.to/epassaro/caching-anaconda-environments-in-github-actions-5hde

.github/workflows/release.yml

@@ -23,7 +23,7 @@ jobs:
       id-token: write
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Set up Python 3.10

.github/workflows/tests.yml

@@ -28,7 +28,7 @@ jobs:
           - macos-latest
           - windows-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Set up Python ${{ matrix.py }}