Update SDK constraint and remove unneeded dependency overrides (#119)

diff --git a/pubspec.yaml b/pubspec.yaml
index 4eee836..d839231 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 homepage: https://github.com/dart-lang/csslib
 
 environment:
-  sdk: '>=2.11.0-0.0 <2.11.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
   source_span: ^1.8.0-nullsafety
@@ -13,7 +13,3 @@
 dev_dependencies:
   pedantic: ^1.10.0-nullsafety
   test: ^1.16.0-nullsafety.2
-
-dependency_overrides:
-  analyzer: 0.40.0
-  html: 0.14.0+3