Removes --causal-async-stacks from rule dart_snapshot_aot

[ghost]

Description

The --causal-async-stacks flag is in the process of being phased out in favour of --lazy-async-stack which will become the new default.

Related Issues

dart-lang/sdk#39525

Checklist

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• I read the [contributor guide] and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation.
• All existing and new tests are passing.
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Breaking Change

Did any tests fail when you ran them? Please read [handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.