This commit removes the 5.4-to-6.0 upgrade guide and all references to it.
It mainly removes references to the Enable Consistent Topology Updates page,
which was added as enabling the feature was optional.
In rare cases, when a reference to that page is necessary,
the internal link is replaced with an external link to version 6.0.
Especially the Handling Cluster Membership Change Failures page was modified
for troubleshooting purposes rather than removed.
32fa5aa938
