Use latest actions/cache for GitHub workflow (#21)

Author: joshmcrae

.github/workflows/tests.yml

@@ -31,7 +31,7 @@ jobs:
 
       - name: Cache Composer packages
         id: composer-cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: vendor
           key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}