A few places in the documentation had mentioned 6.7 as the version to
upgrade from, when doing an upgrade to 7.0. While this is technically
possible, this commit will replace all those mentions to 6.8, as this is
the latest version with the latest bugfixes, deprecation checks and
ugprade assistant features - which should be the one used for upgrades.
Co-Authored-By: James Rodewig <james.rodewig@elastic.co>