Put WRN_InterceptsLocationAttributeUnsupportedSignature into .NET 9 warning wave

[RikkiGibson]

Follow up to #76312
Closes #76641

#76641 (comment)

It was not our goal to make generator authors service the .NET 8 versions of their generators to start using InterceptableLocation. The goal was just to get people moved to the new way for .NET 9 and up. Our plan is to leave in support for the "old way" in the SDK, for as long as .NET 8 is supported. At the time .NET 8 goes out of support, we will fully remove the support for the "old way" from the newest SDK.

I think the resolution here should be to adjust the compiler so that warning InterceptsLocationAttribute(string, int, int) is not supported is only reported when targeting .NET 9 or later.

[RikkiGibson]

@cston @dotnet/roslyn-compiler for review of a small fix

[AlekseyTs]

LGTM (commit 2)