Migrate to react-router 6

[oharsta]

See pull request. Some additional information:

• https://medium.com/@soheiln1234/migrating-gradually-from-react-router-dom-v5-to-v6-a-comprehensive-guide-a1bd226c8470
• https://github.com/remix-run/react-router/issues/8146

[oharsta]

See also https://github.com/OpenConext/OpenConext-manage/blob/master/manage-gui/src/utils/RouterBackwardCompatability.js for a different approach. Do note that this will need changing all history references to navigate.