chore: enable PHP 8.6 testing

.github/workflows/tests.yml

@@ -31,10 +31,9 @@ jobs:
       matrix:
         php-versions: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5']
         experimental: [false]
-# uncomment this to start testing on development release
-#        include:
-#          - php-versions: '8.6' # development release, things can break
-#            experimental: true
+        include:
+          - php-versions: '8.6' # development release, things can break
+            experimental: true
     env:
       extensions: gd, sqlite3
       extensions-cache-key-name: phpextensions