GA: Try again with PHP 8.1

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [7.3, 7.4, 8.0]
+        php: [7.3, 7.4, 8.0, 8.1]
 
     name: PHP ${{ matrix.php }}/Linux
 
@@ -31,6 +31,10 @@ jobs:
       - name: Setup problem matchers
         run: echo "::add-matcher::${{ runner.tool_cache }}/phpunit.json"
 
+      - name: Fix PHPUnit for PHP8.1
+        run: composer config platform.php 8.0
+        if: matrix.php >= 8.1
+
       - name: Setup composer
         run: |
             cp composer.json-dist composer.json