Remove "Beginning in C#" phrase for older versions

[BillWagner]

Throughout the C# guide, there are phrases like "Beginning in C# N", or simply "In C# N".

In some cases, the version referenced goes all the way back to C# 3.

I think we should remove this phrasing for all versions prior to C# 9. (Effectively keep 9, 10, and 11). Then, continue rolling forward to keep only the last 3 versions.