Desktop GL OpenGL 4.6 Support

Intent

Add the capability to initialize an OpenGL 4.6 context instead of a 2.1 context.

Motivation

I was motivated to work on this so I could use RenderDoc for debugging.

I've already achieved this messily, it takes relatively few changes, mostly ignoring capability checks on a 4.6 context and changing some function loads because previously extension and optional support features have become core capability.

I'm mostly asking if there is a preferred way this might be implemented from a User's point of view (how would you prefer a developer specifies the opengl version they would like monogame to initialize with), and whether there is an interest in this capability?

If there is interest and an idiomatic way it could be implemented, I'd be happy to fiddle with my current solution to be more presentable and less of a breaking change and submit a PR for consideration