Prepare to publish (dart-lang/test_process#24)

Fix the version in the changelog and remove redundant entry. Migrating
to null safety implies changing the SDK constraint.

pkgs/test_process/CHANGELOG.md

diff --git a/pkgs/test_process/CHANGELOG.md b/pkgs/test_process/CHANGELOG.md
index 97bf110..3fcbbe9 100644
--- a/pkgs/test_process/CHANGELOG.md
+++ b/pkgs/test_process/CHANGELOG.md
@@ -1,7 +1,6 @@
-## 2.0.0-nullsafety.0
+## 2.0.0
 
 * Migrate to null safety.
-* Require Dart >=2.12.
 
 ## 1.0.6