Make use of the new ObsoletedOSPlatformAttribute in .NET 7

[rolfbjarne]

Ref: dotnet/runtime#68557
Note that the attribute was renamed after it was added, to ObsoletedOSPlatformAttribute (dotnet/runtime#72970)

I think our change is mostly changing our [Obsolete (...)] attributes for .NET to use the new ObsoletedOSPlatform [...] attribute.

We should also likely document how we map Apple's availability attributes to .NET's availability attributes, because it's not entirely obvious:

Apple	.NET
Introduced	SupportedOSPlatform ("ios15.0")
Obsoleted	UnsupportedOSPlatform ("ios16.0")
Deprecated	ObsoletedOSPlatform ("ios17.0")
Unavailable	UnsupportedOSPlatform ("ios")

Additionally we should use the new Message property on the UnsupportedOSPlatform attribute (instead of using an Obsolete attribute to give this information).

[chamons]

So looking at this, I think there are two major parts, one very cheap and one that will require some manual work:

• For NET6 generator targets we should be able to generate the new ObsoletedOSPlatformAttribute attribute pretty easily, and I'll take a stab.
• However, we have a non-trivial amount of non-generator processed code, which might have obsolete attributes that need to be converted by hand. I expect the number is too small to need a pass with mellite, and will just need a nullability like pass.

[rolfbjarne]

BTW this needs to target the net7.0 branch (not main)

[rolfbjarne]

I think 2. should be fairly easy to grep for:

$ git grep Obsolete.*Starting
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveInterruption' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveInterruption' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveAudioRouteChange' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveAudioRouteChange' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveAudioRouteChange' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
src/AVFoundation/Events.cs:             [Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveAudioRouteChange' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
...

and the change would be to change this:

xamarin-macios/src/AVFoundation/Events.cs

Lines 369 to 371 in aa8ded8

	#if IOS
	[Obsolete ("Starting with ios6.0 use 'AVAudioSession.Notification.ObserveInterruption' instead.", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
	#endif

to:

[ObsoletedOSPlatform ("ios6.0", "Use 'AVAudioSession.Notification.ObserveInterruption' instead.")]

(and removing the #if IOS condition)

[chamons]

I believe this is all complete!