chore: update actions cache v4

Author: okunator

.github/workflows/tests.yml

@@ -45,7 +45,7 @@ jobs:
 
       - name: Load cached venv
         id: cached-poetry-dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: .venv
           key: venv-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}