* [DOCS] First pass at upgrade updates for 7.0. * [DOCS] Updates X-Pack terminology * [DOCS] Incorporated feedback from lcawl.
Tweak the upgrade instructions for moving from pre-6.3-with-x-pack to post-6.3-default distribution. Specifically, you have to remove the x-pack plugin before upgrading because 6.4 doesn't understand how to remove it. Relates to #34307