arcaflow-plugins-incubator README.md software version requirements

[webbnh]

The README.md file contains two references to software versions which should be updated:

• The base container image version requirement is documented as CentOS Stream 8; however, this version is now past EoL, and Arcaflow now requires CentOS Stream 9. Should we be explicitly requiring CentOS, or should we instead be requiring something like arcaflow-plugin-baseimage-python-osbase which will be updated over time?
• The required version of Python is 3.10, however, as of this writing, Arca supports 3.9 and uses it internally, so the README.md should probably match that (and, somehow, we need to remember that this requirement is here and update it when Arcaflow's support changes...)