Add CentOS/RHEL 9 support

Add CentOS/RHEL 9 support. Without this change puppet-systemd will
likely fail with the following error:

Evaluation Error: Error while evaluating a Function Call, Class[Systemd]:
expects a value for parameter 'accounting'
(file: /etc/puppet/modules/systemd/manifests/service_limits.pp, line: 40, column: 3)

Tested on Centos 9 Stream.

Signed-off-by: Michele Baldessari <michele@acksyn.org>