Require yum on RHEL 8+ based systems (oamg#1281)

The package manager on RHEL 8+ is dnf but for backwards compatibility
reason a yum rpm is available to create symlinks pointing to dnf.

If the yum package is missing, convert2rhel fails because it calls yum.

Having yum as a dependency of convert2rhel on RHEL 8+ based systems will
ensure that yum will be available (unless someone intentionally removes
yum after the convert2rhel installation but then ¯\_(ツ)_/¯).

packaging/convert2rhel.spec

@@ -44,6 +44,8 @@ Requires:       python%{python_pkgversion}-setuptools
 Requires:       python%{python_pkgversion}-six
 %if 0%{?rhel} && 0%{?rhel} >= 8
 Requires:       dnf
+# The yum rpm mainly takes care of creating symlinks to dnf. We need it since we call yum in convert2rhel.
+Requires:       yum
 # dnf-utils includes yumdownloader and package-cleanup we use
 Requires:       dnf-utils
 Requires:       grubby