[WinForms] `MenuMerge` enum deprecated - mark as obsolete in 3.1?

[RussKie]

In .NET Core 3.1 we have removed a bunch of obsolete API in dotnet/winforms#2157. It looks like we've missed MenuMerge. However it has been removed in .NET 5, when we track-merged the branches.

I suppose we should indicate that this type is obsolete, and is getting removed. What is the right way to indicate this?

[gewarren]

@RussKie If it has the ObsoleteAttribute attribute, then it will automatically get flagged with a Warning box, for example: https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.scriptmanager.scriptpath?view=netframework-4.8

Otherwise, you can add a note to the Remarks section, for example: https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.design.templateeditingverb?view=netframework-4.8#remarks

[gewarren]

Since we can't go back and add ObsoleteAttribute for .NET Core 3.1, we should just add a comment to the remarks that this API was removed in .NET 5.0.