Update flutter to dartdoc 6.3.0 and hide Icons implementation from doc pages (#128442)

This updates dartdoc to 6.3.0.  Release notes are available, here:

https://github.com/dart-lang/dartdoc/releases/tag/v6.3.0

Most important for Flutter are the reduction in the size of generated HTML files (dart-lang/dartdoc#3384) and a new dartdoc directive to hide constant implementations from indicated classes (dart-lang/dartdoc#3398), which fixes the longstanding issue (dart-lang/dartdoc#2657).

I've also added the api documentation zip to `.gitignore` and the `{@hideConstantImplementations}` dartdoc directive to the motivating example.  A screenshot:

![Screenshot 2023-06-07 at 9 54 58 AM](https://github.com/flutter/flutter/assets/14116827/1ad9c1f0-b224-462f-a8e3-706d9858f0d8)

I assert that this change to icons.dart should be test-exempt as existing tests cover whether or not dartdoc directives are recognized or are leaking into HTML, and the impact of adding the directive was tested in dart-lang/dartdoc#3398.

.gitignore

@@ -32,6 +32,7 @@
 /dev/bots/android_tools/
 /dev/devicelab/ABresults*.json
 /dev/docs/doc/
+/dev/docs/api_docs.zip
 /dev/docs/flutter.docs.zip
 /dev/docs/lib/
 /dev/docs/pubspec.yaml

dev/bots/docs.sh

@@ -20,7 +20,7 @@ function generate_docs() {
     # Install and activate dartdoc.
     # When updating to a new dartdoc version, please also update
     # `dartdoc_options.yaml` to include newly introduced error and warning types.
-    "$DART" pub global activate dartdoc 6.2.2
+    "$DART" pub global activate dartdoc 6.3.0
 
     # Install and activate the snippets tool, which resides in the
     # assets-for-api-docs repo:

packages/flutter/lib/src/cupertino/icons.dart

@@ -59,6 +59,7 @@ import 'package:flutter/widgets.dart';
 /// See also:
 ///
 ///  * [Icon], used to show these icons.
+/// {@hideConstantImplementations}
 @staticIconProvider
 abstract final class CupertinoIcons {
   /// The icon font used for Cupertino icons.

packages/flutter/lib/src/material/icons.dart

@@ -149,6 +149,7 @@ final class PlatformAdaptiveIcons implements Icons {
 ///  * [IconButton]
 ///  * <https://material.io/resources/icons>
 ///  * [AnimatedIcons], for the list of available animated Material Icons.
+/// {@hideConstantImplementations}
 @staticIconProvider
 abstract final class Icons {
   /// A set of platform-adaptive Material Design icons.