RHEL-9 is still using ansible-core 2.14.

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.

As ansible-freeipa roles do not support version 2.8 anymore, change the
minimum supported version to 2.13, which is the currently minimum
available and supported Ansible version.

This patch fixes documentation on all plugin READMEs, spec file and
module templates.

There is a new netgroup management module placed in the plugins folder:

    plugins/modules/ipanetgroup.py

The netgroup module allows to ensure presence or absence of netgroup
and manage netgroup members.

Here is the documentation for the module:

    README-netgroup.md

New example playbooks have been added:

    playbooks/netgroup/netgroup-absent.yml
    playbooks/netgroup/netgroup-member-absent.yml
    playbooks/netgroup/netgroup-member-present.yml
    playbooks/netgroup/netgroup-present.yml

New tests for the module:

    tests/netgroup/test_netgroup.yml
    tests/netgroup/test_netgroup_client_context.yml
    tests/netgroup/test_netgroup_member.yml
    tests/netgroup/test_netgroup_member_absent.yml
    tests/netgroup/test_netgroup_member_case_insensitive.yml

Signed-off-by: Denis Karpelevich <dkarpele@redhat.com>