Flip 6.8 docs to unreleased
This version is not yet released so this commit flips the docs state to
unreleased.
release-state can be: released | prerelease | unreleased
//////////
-:release-state: released
+:release-state: unreleased
:issue: https://github.com/elastic/elasticsearch/issues/
:ml-issue: https://github.com/elastic/ml-cpp/issues/
