Update actions/cache action to v4 (#41)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -22,14 +22,14 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
       - name: Cache mix dependencies
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: deps
           key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
           restore-keys: |
             ${{ runner.os }}-mix-
       - name: Cache _build
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: _build
           key: ${{ runner.os }}-build-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}