[generator] Add support for [ObsoletedOSPlatform].

[jpobst]

Context: dotnet/android#7234

Add support for a new lang-features=obsoleted-platform-attributesgenerator option.

When used, for API that was obsoleted in API levels greater than our .NET 7 minimum (API 21), we will generate .NET 7's new [ObsoletedOSPlatform] attribute instead of[Obsolete]:

// New[global::System.Runtime.Versioning.ObsoletedOSPlatform("android22.0",@"This class is obsoleted in this android platform")]publicpartialclassCookieSpecParamBean:Org.Apache.Http.Params.HttpAbstractParamBean{ ...}// Previous[global::System.Obsolete(@"This class is obsoleted in this android platform")]publicpartialclassCookieSpecParamBean:Org.Apache.Http.Params.HttpAbstractParamBean{ ...}

This is useful for a .NET 7+ context because we always compile against a "latest" Mono.Android, even if you are targeting an earlier version. For example, this causes the above class to throw an obsolete warning even if you are using $(SupportedOSPlatformVersion)=21.

Companion XA test PR: dotnet/android#7266