fix assumption that 6.7 is last 6.x release (#42255)

docs/reference/upgrade.asciidoc

@@ -7,8 +7,8 @@
 process so upgrading does not interrupt service. Rolling upgrades are supported:
 
 * Between minor versions
-* From 5.6 to 6.7
-* From 6.7 to {version}
+* From 5.6 to 6.8
+* From 6.8 to {version}
 
 {es} can read indices created in the previous major version. If you
 have indices created in 5.x or before, you must reindex or delete them
@@ -21,7 +21,7 @@ When upgrading to a new version of {es}, you need to upgrade each
 of the products in your Elastic Stack. For more information, see the
 {stack-ref}/upgrading-elastic-stack.html[Elastic Stack Installation and Upgrade Guide].
 
-To upgrade directly to {version} from 6.6 or earlier, you must shut down the
+To upgrade directly to {version} from 6.7 or earlier, you must shut down the
 cluster, install {version}, and restart. For more information, see
 <<restart-upgrade, Full cluster restart upgrade>>.
 

docs/reference/upgrade/cluster_restart.asciidoc

@@ -1,11 +1,11 @@
 [[restart-upgrade]]
 == Full cluster restart upgrade
 
-To upgrade directly to {es} {version} from versions 6.0-6.6, you must shut down
+To upgrade directly to {es} {version} from versions 6.0-6.7, you must shut down
 all nodes in the cluster, upgrade each node to {version}, and restart the cluster.
 
 NOTE: If you are running a version prior to 6.0,
-https://www.elastic.co/guide/en/elastic-stack/6.7/upgrading-elastic-stack.html[upgrade to 6.7]
+https://www.elastic.co/guide/en/elastic-stack/6.8/upgrading-elastic-stack.html[upgrade to 6.8]
 and reindex your old indices or bring up a new {version} cluster and
 <<reindex-upgrade-remote, reindex from remote>>.
 

docs/reference/upgrade/rolling_upgrade.asciidoc

@@ -10,13 +10,13 @@ running the older version.
 Rolling upgrades are supported:
 
 * Between minor versions
-* https://www.elastic.co/guide/en/elastic-stack/6.7/upgrading-elastic-stack.html[From 5.6 to 6.7]
-* From 6.7 to {version}
+* https://www.elastic.co/guide/en/elastic-stack/6.8/upgrading-elastic-stack.html[From 5.6 to 6.8]
+* From 6.8 to {version}
 
-Upgrading directly to {version} from 6.6 or earlier requires a
+Upgrading directly to {version} from 6.7 or earlier requires a
 <<restart-upgrade, full cluster restart>>.
 
-To perform a rolling upgrade from 6.7 to {version}:
+To perform a rolling upgrade from 6.8 to {version}:
 
 . *Disable shard allocation*.
 +