Update deprecated activeColor to activeThumbColor in Learn Flutter example code

[miinhho]

Description of what this PR is changing or adding, and why:
Replaces the deprecated activeColor property with activeThumbColor in Learn Flutter example code (https://docs.flutter.dev/get-started/fundamentals/user-input#switch) to help new Flutter developers learn current best practices and avoid using deprecated APIs.

Issues fixed by this PR (if any):
#12690

PRs or commits this PR depends on (if any):

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[gemini-code-assist]

Summary of Changes

Hello @miinhho, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates an example code snippet in the Flutter documentation to reflect current best practices. It specifically addresses the deprecation of the activeColor parameter in the Switch widget by replacing it with the recommended activeThumbColor. This change helps new developers learn the most up-to-date API usage and avoids exposing them to outdated features.

Highlights

• Deprecated API Update: Replaced the deprecated activeColor parameter with activeThumbColor in a Flutter Switch widget example within the documentation.
• Improved Learning Experience: Ensures new Flutter developers learn current best practices and avoid using deprecated APIs, aligning the example code with the latest Flutter standards.

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[gemini-code-assist]

Code Review

This pull request correctly replaces the deprecated activeColor property with activeThumbColor in the Switch widget example. To provide a more complete and visually consistent example, I've suggested also setting activeTrackColor, as activeColor previously affected both the thumb and the track.

[sfshaza2]

@ericwindmill, can you verify

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 82067a3):

https://flutter-docs-prod--pr12691-main-kg8dftjh.web.app

[sfshaza2]

lgtm