GH Actions: update action runner versions

GitHub has deprecated the Node 16 action runner environment and will soon stop supporting it. Both the `actions/checkout` as well as `ramsey/composer-install` have released new versions for this.

Updating to those newer versions will prevent the CI from breaking once support for Node 16 is stopped completely.

Author: jrfnl

.github/workflows/main.yml

@@ -25,10 +25,10 @@ jobs:
         php-version: '${{ matrix.php }}'
         tools: composer
         extensions: 'xdebug'
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Install Composer dependencies & cache dependencies
-      uses: "ramsey/composer-install@v2"
+      uses: "ramsey/composer-install@v3"
       with:
         composer-options: "--prefer-dist"
         custom-cache-key: "{{ runner.os }}-composer-${{ matrix.php }}"