Automate updating Fedora version in docs
In many places, a Fedora version number is used as and example,
in strings like Fedora 34, f34, fc34.
The docs read better if the version is always the current release,
or perhaps one behind.
AsciiDoc attributes are used to define
the current and previous OS version
as well as the current EPEL version
in one place.
The attributes are then referred to
in all examples that need those values.
Resolves #28