Make README clearer with regard to version references

[d-w-moore]

The language here shifts to versions of the iRODS server... but we can tweak it later.

Originally posted by @trel in https://github.com/irods/python-irodsclient/pull/524#discussion_r1581282248

For expediency in release schedule, some potentially confusing language was allowed to remain in the python-irodsclient (PRC) README esp. with regard to discussions of server/PRC versions