[AVFoundation/introspection] Fix availability attributes / introspection for watchOS 5.0. Fixes #xamarin/maccore@2420.

[rolfbjarne]

Fixes https://github.com/xamarin/maccore/issues/2420.

[rolfbjarne]

Test failures are unrelated:

• monotouch-test/Mac [dotnet]/Debug (CoreCLR) [dotnet]: Fixed in [ObjCRuntime] Unwrap TargetInvocationExceptions we get when invoking methods using reflection. #11733 and [tests] Adjust MX8029_b and MX8033 tests to cope with slightly different exception message output for CoreCLR. #11732.
• monotouch-test/Mac [dotnet]/Debug (CoreCLR, static registrar) [dotnet]: Fixed in [ObjCRuntime] Unwrap TargetInvocationExceptions we get when invoking methods using reflection. #11733 and [tests] Adjust MX8029_b and MX8033 tests to cope with slightly different exception message output for CoreCLR. #11732.
• monotouch-test/iOS Unified 32-bits - simulator/Debug: monotouch-test / iOS Unified 32-bits - simulator / Debug -> multiple failures #11504

[vs-mobiletools-engineering-service2]

✅ [PR Build] Tests passed on Build. ✅

Tests passed on Build.

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✅ API Diff from stable

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• iOS vsdrops gist
• macOS vsdrops gist
• macCat vsdrops gist
• macCatiOS gist
• tvOS vsdrops gist
• watchOS vsdrops gist

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ℹ️ Generator Diff (please review changes)

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🎉 All 83 tests passed 🎉

Pipeline on Agent XAMBOT-1094.BigSur'
Merge d1ac246 into dc30bdf