[ui/accessibility] Update web accessibility docs for Flutter 3.50

[kevmoo]

Updates web accessibility and testing documentation for the Flutter 3.50 WCAG 2.1 AA improvements and removes a duplicated sentence in web-accessibility.md:

• Documents <input type="range" role="slider"> semantics (Slider, RangeSlider, and SemanticsRole.slider), named route role="region" landmarks, focus preservation across semantic role changes, and Escape tooltip dismissal.
• Documents maximum-scale=5.0 viewport pinch-to-zoom, iOS Safari 16px input font-size floor, <flutter-view> scoping for <flt-semantics-placeholder>, and runtime debugShowSemanticsNodes: true configuration in flutter_bootstrap.js.

Fixes #13963

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 530b705):

https://flutter-docs-prod--docs-pr13987-web-a11y-13963-t75hg5mc.web.app

[gemini-code-assist]

Code Review

This pull request updates the Flutter web accessibility documentation to include instructions for enabling semantic node visualization at runtime via flutter_bootstrap.js, details on mobile browser scoping for the accessibility button, and new features introduced in Flutter 3.50 (such as enhanced slider semantics, landmarks, focus preservation, and viewport zoom configurations). The review feedback correctly identifies two misleading technical descriptions: it recommends clarifying that namesRoute is a boolean property rather than a label string to prevent compilation errors, and suggests correcting the non-standard reference to the standard viewport tag with id="flt-viewport".