Rename 'clean' Gradle task that required Flutter and has been breaking nightly build

[alexeyinkin]

This PR renames the Playground's clean Gradle task to cleanFlutter so it does not automatically start when clean is run in all sub-projects.

A task flutterClean in another component is also renamed to cleanFlutter for consistency.

The new naming is in line with cleanPython tasks found elsewhere in the project.

• Resolves [Bug]: Nightly Snapshot build is broken #23514

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[alexeyinkin]

R: @damccorm

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[damccorm]

LGTM, thanks