tolerate test failures in the PHP development release

at this time, guzzle, dependency of google cloud storage library, raises deprecation warnings in PHP 8.4, which caused the tests to be considered failed

.github/workflows/tests.yml

@@ -16,11 +16,16 @@ jobs:
       run: composer install --prefer-dist --no-dev
 
   PHPunit:
+    name: PHP ${{ matrix.php-versions }} unit tests on ${{ matrix.operating-system }}
     runs-on: ubuntu-latest
+    continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-        php-versions: ['7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
-    name: PHP ${{ matrix.php-versions }} unit tests on ${{ matrix.operating-system }}
+        php-versions: ['7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
+        experimental: [false]
+        include:
+          - php-versions: '8.4' # development release, things can break
+            experimental: true
     env:
       extensions: gd, sqlite3
       extensions-cache-key-name: phpextensions