Add conflict with doctrine/dbal 3

doctrine/dbal is an optional dependency of laravel/framework, and v3 of
the DBAL has been released recently, but obviously, Laravel was never
meant to be compatible with that version.
Users still using Laravel 6 and updating their dependencies get confused
and file invalid reports on the DBAL, see for instance
doctrine/dbal#4439
or
doctrine/dbal#4757

Fixes #24271

composer.json

@@ -74,6 +74,7 @@
         "illuminate/view": "self.version"
     },
     "conflict": {
+        "doctrine/dbal": ">=3.0",
         "tightenco/collect": "<5.5.33"
     },
     "require-dev": {