mark `null_check_on_nullable_type_parameter` stable (#3145)

diff --git a/lib/src/rules/null_check_on_nullable_type_parameter.dart b/lib/src/rules/null_check_on_nullable_type_parameter.dart
index 55292fc..34d311e 100644
--- a/lib/src/rules/null_check_on_nullable_type_parameter.dart
+++ b/lib/src/rules/null_check_on_nullable_type_parameter.dart
@@ -50,7 +50,7 @@
           name: 'null_check_on_nullable_type_parameter',
           description: _desc,
           details: _details,
-          maturity: Maturity.experimental,
+          maturity: Maturity.stable,
           group: Group.style,
         );