Prepare for dartdoc 6.3.0. (#3430)

* Update changelog and pubspec for dartdoc 6.3.0.

* Missed a crucial change in the changelog

Author: jcollins-g

CHANGELOG.md

@@ -1,7 +1,10 @@
-## 6.2.2-dev
+## 6.3.0
 * Add a hideImplementations directive to disable showing constant
   implementations alongside one-line docs on class pages (#3398)
 * Update highlight.js to 11.8.0 for Dart 3 syntax highlight support. (#3416)
+* Split sidebars out into separate HTML files (#3384)
+* Update several required dependency versions, switch to Dart
+  language version 3.0, and several internal refactors. (various)
 
 ## 6.2.2
 * Add chips to class pages for class modifiers (#3401)

dartdoc_options.yaml

@@ -1,4 +1,4 @@
 dartdoc:
   linkToSource:
     root: '.'
-    uriTemplate: 'https://github.com/dart-lang/dartdoc/blob/v6.2.2-dev/%f%#L%l%'
+    uriTemplate: 'https://github.com/dart-lang/dartdoc/blob/v6.3.0/%f%#L%l%'