generator: increase supported analyzer to v3

Author: greenrobot-team

generator/pubspec.yaml

@@ -9,7 +9,7 @@ environment:
 
 dependencies:
   objectbox: 1.3.0
-  analyzer: '>=1.1.0 <3.0.0'
+  analyzer: '>=1.1.0 <4.0.0'
   build: ^2.0.0
   collection: ^1.15.0
   dart_style: ^2.0.0