Prepare for release 2.51.0 (#9454)

Author: srawlins

packages/devtools_app/lib/devtools.dart

@@ -10,4 +10,4 @@
 /// Note: a regexp in the `dt update-version' command logic matches the constant
 /// declaration `const version =`. If you change the declaration you must also
 /// modify the regex in the `dt update-version' command logic.
-const version = '2.51.0-dev.0';
+const version = '2.51.0';

packages/devtools_app/pubspec.yaml

@@ -7,7 +7,7 @@ publish_to: none
 
 # Note: this version should only be updated by running the 'dt update-version'
 # command that updates the version here and in 'devtools.dart'.
-version: 2.51.0-dev.0
+version: 2.51.0
 
 repository: https://github.com/flutter/devtools/tree/master/packages/devtools_app