Updated the threading notice for platform channel responses. (#7901)

This updates the guidelines about threading and the responses to
platform channels. Once the following PRs are on `main` all official
platforms (minus web where it doesn't make sense) support thread-safe
responses.

issue: flutter/flutter#93945

Do no land until the following are on stable:
1) flutter/engine#37689
1) flutter/engine#37607
1) flutter/engine#36909

## Presubmit checklist
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(Grammarly or similar).
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Co-authored-by: Shams Zakhour (ignore Sfshaza) <[email protected]>
Co-authored-by: Parker Lougheed <[email protected]>

Author: 3 people

src/platform-integration/platform-channels.md

@@ -1222,8 +1222,7 @@ either invoke them from any `Isolate` that is the root
 The handlers for the platform side can execute on the platform's main thread
 or they can execute on a background thread if using a Task Queue.
 You can invoke the platform side handlers asynchronously
-and on any thread when the Task Queue API is available;
-otherwise, they must be invoked on the platform thread.
+and on any thread.
 
 {{site.alert.note}}
   On Android, the platform's main thread is sometimes