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.