Fixing the UI (the focus of this issue) is easy - one string on one line.
The larger concern is that the API for getting statistical information out of Solr has changed in a minor release, in a way that can break clients, and nothing in CHANGES.txt indicated that there was such a change. I understand the desire to remove "HANDLER" from the category names – it's unnecessarily verbose. If it were me making the improvements, I would have left the category names unchanged in 6.x, only making the changes in master.
Users expect that their client code will continue working if they make a minor version upgrade, and for the most part Solr has kept that promise. Sometimes progress requires changes that break user expectations, but in that case, it should be mentioned in the changelog.