README-*: Bump minimum Ansible supported version to 2.15

ansible-freeipa roles do not work with Ansible 2.8 anymore, so the
minimum supported version is changed to 2.15, the oldest supported
Ansible version as of today.

This patch includes the change to the version number in the collection
and all plugin README files. The collection README was also update to
remove text that related only to previous Ansible versions.