use new cmake 3.22 feature to get distro pretty name

- stop setting a version, prefer pretty name instead
- default the pretty name to the data from os-release when run with a
new enough cmake
- when not new enough use the previous plasma$version string as pretty
name
- version is kept around for backwards compat, script code also still
constructs "%1 %2" as final string, i.e. we have a trailing space but it
shouldn't matter in practise