Remove 6.0 breaking changes documentation

In master branch (7.0) we only need to have the 7.0 breaking changes.

Note that we probably need to adjust the messaging in `index.asciidoc` cluster upgrades between 6.x and 7.x.

Follow up for elastic#26099.

Author: dadoonet

docs/reference/migration/index.asciidoc

@@ -19,4 +19,5 @@ As a general rule:
 
 See <<setup-upgrade>> for more info.
 --
-include::migrate_6_0.asciidoc[]
+
+include::migrate_7_0.asciidoc[]