update cache action to v4 (#712)

Author: dionhaefner

.github/workflows/test-all.yml

@@ -40,7 +40,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Restore pip cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         id: pip-cache
         with:
           path: ~/.cache/pip
@@ -49,7 +49,7 @@ jobs:
             ${{ matrix.os }}-pip-${{ matrix.python-version }}-
 
       - name: Restore PyOM2 build cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         id: pyom2-cache
         with:
           path: ${{ env.PYOM2_DIR }}/py_src/*.so

.github/workflows/test-conda.yml

@@ -33,7 +33,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Cache conda
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ~/conda_pkgs_dir
           key: