Removed brand references from MenuAnchor.dart

[davidhicks980]

Rephrases identifier from _isApple to _isCupertino.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
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[auto-submit]

auto label is removed for flutter/flutter/148432, due to This PR has not met approval requirements for merging. The PR author is not a member of flutter-hackers and needs 1 more review(s) in order to merge this PR.

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[gspencergoog]

[auto-submit]

auto label is removed for flutter/flutter/148432, Failed to merge flutter/flutter/148432 with Pull request flutter/flutter/148432 could not be merged: You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more information..

[auto-submit]

auto label is removed for flutter/flutter/148432, Failed to merge flutter/flutter/148432 with Pull request flutter/flutter/148432 could not be merged: You're not authorized to push to this branch. Visit https://docs.github.com/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches for more information..

[gspencergoog]

Hi @davidhicks980 Thanks for the PR. There's only one problem: this is a PR against the main branch, not against master. We're still in the process of migrating to main and it doesn't yet allow us to commit on main, only master. Could you please create a new PR against master and tag me and @victorsanni on it so we can get it committed?

[davidhicks980]

See #148760