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

.github/workflows/docs.yaml

@@ -10,7 +10,7 @@ jobs:
     if: github.repository == 'PHPMailer/PHPMailer'
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           fetch-depth: 1
       - name: Build Docs

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Set up PHP
         uses: shivammathur/setup-php@v2
@@ -50,7 +50,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install PHP
         uses: shivammathur/setup-php@v2
@@ -100,7 +100,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Set coverage variable
         id: set_cov