Bump to actions/checkout@v3

Related to this deprecation of NodeJS 12 for GHA actions:

https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/

While it doesn't affect to this repository, let's bump to the new v3 version now so we keep it updated.

Author: stronk7

.github/workflows/phpcs.yml

@@ -15,7 +15,7 @@ jobs:
           - php: 7.4
     steps:
       - name: Check out repository code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Setup PHP ${{ matrix.php }}
         uses: shivammathur/setup-php@v2