Fix README

[pwalczysko]

Fixes #375 - the README is outdated in 2 points:

1. It does not specify that the playbooks are working with CentOS 7 OS.
2. It points to ome-ansible-molecule which is not used anymore

@MartinDerHettich

[pwalczysko]

@sbesson imho, although the tests are failing (I believe because the CentOS 7 mirror cannot be found as CentOS is EOF), this PR should prevent needless install attempts such as #375

[sbesson]

I am no longer involved in these playbooks so you might want a review from an active maintainer.

If the goal is to eventually update this repository with new RHEL9 playbooks, the warning makes sense to me. If the playbooks are not expected to be updated, it might make sense to go one step further and mark this repository as read-only.

[joshmoore]

I'm going to merge as an immediate improvement on the former situation. There are definitely a number of additional options we can then consider:

• update the playbooks with the in-progress RHEL9 versions
• separate the playbooks into separate versioned directories
• archive the repo with a note pointing to the new starting point (perhaps just the roles themselves)