Change deprecated action/cache version (documentation)

Author: Belval

.github/workflows/documentation.yml

@@ -33,7 +33,7 @@ jobs:
         run: echo "dir=$(pip cache dir)" >> "$GITHUB_OUTPUT"
 
       - name: Cache dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ steps.pip-cache.outputs.dir }}
           key: ${{ runner.os }}-pip-${{ hashFiles('**/extras/docs.txt') }}