Update the Cloudsmith repositories in the quickstart section in the ARM

We should document the new repositories kea-dev or kea-stable at https://kea.readthedocs.io/en/latest/arm/quickstart.html#quick-start-guide-using-native-packages.

It would be nice if we could add some simple logic in doc/sphinx/conf.py to automatically determine the repository that needs to be used according to the Kea version before passing it to rst_prolog. So the ARM for 2.6.0 would say kea-stable and the ARM for 2.7.0 would say kea-dev.