UIKit introspection test failures on macOS 11.5

[rolfbjarne]

Introspection.iOSApiCtorInitTest
    [FAIL] UIKit.UIContentUnavailableButtonProperties : Handle
    [FAIL] UIKit.UIContentUnavailableImageProperties : Handle
    [FAIL] UIKit.UIContentUnavailableTextProperties : Handle
    [FAIL] DefaultCtorAllowed :   3 potential errors found in 1535 default ctor validated:
UIKit.UIContentUnavailableButtonProperties : Handle
UIKit.UIContentUnavailableImageProperties : Handle
UIKit.UIContentUnavailableTextProperties : Handle

  Expected: 0
  But was:  3
           at Introspection.ApiCtorInitTest.DefaultCtorAllowed() in /Users/builder/azdo/_work/2/s/xamarin-macios/tests/introspection/ApiCtorInitTest.cs:line 304
    [PASS] DesignatedInitializer
    [PASS] ShouldNotExposeDefaultCtorTest
Introspection.iOSApiProtocolTest
    [PASS] Coding
    [PASS] Copying
    [FAIL] GeneralCase : 3 types do not really conform to the protocol interfaces: [FAIL] Could not load UIKit.UIContentUnavailableButtonProperties
        [FAIL] Could not load UIKit.UIContentUnavailableImageProperties
        [FAIL] Could not load UIKit.UIContentUnavailableTextProperties
        [FAIL] Could not load UIKit.UIContentUnavailableButtonProperties
        [FAIL] Could not load UIKit.UIContentUnavailableImageProperties
        [FAIL] Could not load UIKit.UIContentUnavailableTextProperties
           at Introspection.ApiBaseTest.AssertIfErrors(String s, Object[] parameters) in /Users/builder/azdo/_work/2/s/xamarin-macios/tests/introspection/ApiBaseTest.cs:line 136
           at Introspection.ApiProtocolTest.GeneralCase() in /Users/builder/azdo/_work/2/s/xamarin-macios/tests/introspection/ApiProtocolTest.cs:line 860
    [PASS] MutableCopying
    [PASS] SecureCoding
    [PASS] SupportsSecureCoding

Ref: #19347 (comment)