Upgrade language version to 3.0 (#3822)

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 447ee06..d93fc33 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,7 +1,7 @@
 name: pub
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '^3.0.0-0'
 
 dependencies:
   # Note: Pub's test infrastructure assumes that any dependencies used in tests