Feature: doc: Drop support for rolling upgrades from versions < 2.0.0

As of this commit, rolling upgrades from a Pacemaker version less than
2.0.0 are no longer guaranteed to work.

The feature set number is not updated because this doesn't introduce a
compatibility issue with the version of Pacemaker immediately prior to
this commit.

Ref T13

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>