Correct GitHub Action deprecations + Opt into Dependabot (#23)

* fix: update to latest github action versions

* feat: opt into Dependabot for GitHub Action health

Author: iBotPeaches

Diff for: .github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

Diff for: .github/workflows/formats.yml

@@ -18,10 +18,10 @@ jobs:
     steps:
 
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Cache dependencies
-        uses: actions/cache@v1
+        uses: actions/cache@v4
         with:
           path: ~/.composer/cache/files
           key: dependencies-php-${{ matrix.php }}-composer-${{ hashFiles('composer.json') }}

Diff for: .github/workflows/tests.yml

@@ -28,10 +28,10 @@ jobs:
     steps:
 
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Cache dependencies
-        uses: actions/cache@v1
+        uses: actions/cache@v4
         with:
           path: ~/.composer/cache/files
           key: dependencies-php-${{ matrix.php }}-SF${{ matrix.symfony }}-${{ matrix.dependency-version }}-composer-${{ hashFiles('composer.json') }}