GH Actions: be explicit about versions and test against 8.1

This adds an "experimental" build against PHP 8.1 as an early detection system for compatibility issues. This build is allowed to fail.

Author: jrfnl

.github/workflows/run-tests.yml

@@ -21,10 +21,17 @@ jobs:
 
     strategy:
       matrix:
-        php: [8.x, 7.4, 7.3]
+        php: [8.0, 7.4, 7.3]
         dependency-version: [prefer-lowest, prefer-stable]
+        experimental: [false]
+
+        include:
+          - php: '8.1'
+            dependency-version: 'prefer-stable'
+            experimental: true
 
     name: P${{ matrix.php }} - ${{ matrix.dependency-version }}
+    continue-on-error: ${{ matrix.experimental }}
 
     steps:
     - name: Checkout code
@@ -37,9 +44,15 @@ jobs:
         coverage: none
         tools: composer
 
-    - name: Install dependencies
+    - name: Install dependencies - normal
+      if: ${{ matrix.php < 8.1 }}
       run: |
         composer update --${{ matrix.dependency-version }} --prefer-dist --no-progress
 
+    - name: Install dependencies - ignore platform reqs
+      if: ${{ matrix.php >= 8.1 }}
+      run: |
+        composer update --${{ matrix.dependency-version }} --prefer-dist --no-progress --ignore-platform-reqs
+
     - name: Execute Unit Tests
       run: vendor/bin/phpunit