GH Actions: version update for various predefined actions

A number of predefined actions have had major release, which warrant an update the workflow(s).

These updates don't actually contain any changed functionality, they are mostly just a change of the Node version used by the action itself (from Node 14 to Node 16).

Refs:
* https://github.com/actions/checkout/releases

Author: jrfnl

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install PHP
         uses: shivammathur/setup-php@v2
@@ -70,7 +70,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install PHP
         uses: shivammathur/setup-php@v2