Update minimum XBC version support

[virn]

We've been adding version checks every time we have a new feature or change from XBC.
I think it is also time, moving in 3.0 to review and update the minimum version of XBC we check for back compatibility on the framework.
This would also have an impact on our bundle size.

Let me know what you guys think.

cc @mikeybanez @SML-MeSo

[mikeybanez]

A major version should imply breaking changes, so it's an opportunity to wipe the slate clean. Whatever is the current public version of XBC when xjs 3.0.0 is released should be fully supported by 3.0.0.

That means that we need to drop all droppable XBC version checks from all of our methods when we have our feature-locked final candidate for xjs 3.0.0.