chore: adjust dart sdk constraint to ">=2.12.0 <4.0.0"

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 783bc5a..7760ec6 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,7 +9,7 @@
 funding: [https://github.com/sponsors/felangel]
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.12.0 <4.0.0"
 
 dependencies:
   collection: ^1.15.0