[readme][changelog][license] correct typesetting of DuMux
Reviewing the Changelog I found some old-DuMux-style typesetting and quickly went through our README.md, CHANGELOG.md and LICENSE.md to use DuMu<sup>x</sup>
Do we want this to be in the same typesetting everywhere? Then this is the MR. If not, I can drop it and we only adjust the README.md