Checklist for major release v6.0.0

[vgrozdanic]

With a couple of breaking changes about to happen, this issue is a catch all checklist for everything that is planned to be done that breaks compatibility with previous versions, and requires major version to be incremented :

• stringify sub claim - A silenced error for JWT Stateless Authentication during token validation #832
• add security warning in for_user docs, figure out what to do with TokenVerifyView, validate if user is active in TokenRefreshSerializer, TokenRefreshSlidingSerializer - A silenced error for JWT Stateless Authentication during token validation #832
• stop raising DoesNotExist and start raising AuthenticationFailed - fix: Avoid DoesNotExist exception in TokenRefreshSerializer #861