Readme badges and pyproject cleanup

[martinhoyer]

I'm not a friend with rst and have more experience with markdown. Will this render ok on PyPI page and other places? We also need to deal with man generation I presume?
PyPA should be ok based on:

$ twine check dist/tmt-1.40.0.dev2+ga31e8651-py3-none-any.whl --strict
Checking dist/tmt-1.40.0.dev2+ga31e8651-py3-none-any.whl: PASSED

github would look like this:

In pyproject, I've replaced Python 3.11 with 3.13 and removed the unmaintainable comments of what versions are available where.

Pull Request Checklist

• implement the feature