docs: update predictive back guide to reflect default behavior

[lamek]

Rewrote the page to reflect that predictive back is now enabled by default in Flutter, removing instructions for manual app-level transitions setup

#12201

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[gemini-code-assist]

Code Review

This pull request updates the Android predictive back gesture documentation to reflect that the feature is now supported by default in Flutter, removing the manual theme configuration steps. The feedback suggests changing the page title to 'Support the predictive back gesture' for better accuracy, and updating the minimum required Flutter version from 3.22.2 to 3.24 to ensure users do not experience broken transitions, as the default support was introduced in the later version.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 52928e7):

https://flutter-docs-prod--docs-pr13675-fix-12201-uq3346ng.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 52928e7):

https://flutter-dev-230821--www-pr13675-fix-12201-s3957rv2.web.app