[Breaking] Remove `[DataTestMethod]`

[Evangelink]

They are fully interchangeable and [DataTestMethod] as no different value so I suggest to mark it as obsolete (let's use a special diagnostic code to make it easy for people).

[nohwnd]

https://github.com/approvals/ApprovalTests.Net/blob/master/src/ApprovalTests/Namers/UnitTestFrameworks/MsTestDataTestMethodStackTraceParser.cs

Looks like there are some more usages of this as well. Are you sure this won't have impact that outweighs the benefits (are there any, other than having one less class?)

https://grep.app/search?q=DataTestMethodAttribute

[Evangelink]

It will definitely cause warnings but the class is fully pointless so I want to get rid of it in v4 meaning we need to give some transitioning time to users.

[nohwnd]

What is our evidence for being fully pointless? I agree code wise that it does nothing for the test framework, but maybe it has semantic meaning for the consumers and they built their tools depending on this meaning.

[SteveSyfuhs]

Yeah, this is a really annoying change for what appears to be zero value to the consumer. All it's doing is causing churn in test code. Please reconsider how much time and energy you're asking all consumers to invest with this change. It could be as simple as find and replace but it might not be.