macOS: Add @available check at macOS 12 workaround

[cbracken]

Use default mouse event handling behaviour on macOS 13.3.1 onwards. This has two positive effects:

• Avoids the workaround on newer macOS versions where it's unnecessary, thereby giving us confidence that the underlying AppKit issue is fixed and the whole method can later be removed.
• Will be caught by tooling when we drop support for versions of macOS prior to the fixed version.

Issue: flutter/flutter#154063
Issue: flutter/flutter#115015

No tests modified since there is no semantic change, either on versions of macOS where the issue is fixed (and thus the default event handler is correct) or on versions where it is not (and we still use the workaround).

Re-tested manually with the reduced transparency setting on macOS 14.6.1.

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• I listed at least one issue that this PR fixes in the description above.
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[flutter-dashboard]

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[stuartmorgan]

No tests modified since there is no semantic change

Won't the tests you added for this workaround fail if run on 13.3+?

[cbracken]

Based on the presubmit

No tests modified since there is no semantic change

Won't the tests you added for this workaround fail if run on 13.3+?

Based on the presubmit... yes 😂 Looks like I got some more conditionalising to do.

[cbracken]

Conditionalification complete. At least one of various ways I could have done it.

No need for a test exemption cause I've now updated the test.

[jmagman]

Thanks for following up on this!