Fix documentation CI workflow (#945)

Use `actions/cache@v4` and fix cache key.

Author: LandonTClipp

.github/workflows/documentation.yml

@@ -12,9 +12,14 @@ jobs:
       - uses: actions/setup-python@v4
         with:
           python-version: 3.x
-      - uses: actions/cache@v2
+      - name: Get Date
+        id: get-date
+        run: |
+          echo "date=$(/bin/date -u "+%Y%m")" >> $GITHUB_OUTPUT
+        shell: bash
+      - uses: actions/cache@v4
         with:
-          key: ${{ github.ref }}
+          key: ${{ runner.os }}-${{ steps.get-date.outputs.date }}
           path: .cache
       - run: sudo apt-get install -y libcairo2-dev libfreetype6-dev libffi-dev libjpeg-dev libpng-dev libz-dev
       - run: pip install -r docs/requirements.txt