Suggestion

Reference: #1752 (comment)

DefinitelyTyped only tests against and guarantees support for versions up to 2 years old - https://github.com/DefinitelyTyped/DefinitelyTyped#support-window

We talked about this a while ago but never got around to it - let's bump the minimum version in the next major.

This will allow us to delete a bunch of branches from our codebase reducing complexity and maintenance burden.