Set max PHP version.

We won't be supporting PHP8 in 3.x. Several of our dependencies are not
compatible and upgrading them would also require breaking change in
CakePHP. Folks needing PHP8 support will need to upgrade to 4.x

composer.json

@@ -28,7 +28,7 @@
         "source": "https://github.com/cakephp/cakephp"
     },
     "require": {
-        "php": ">=5.6.0",
+        "php": ">=5.6.0,<8.0.0",
         "ext-intl": "*",
         "ext-mbstring": "*",
         "cakephp/chronos": "^1.0.1",