common: use ansible_distribution_major_version in epel repos

common: use ansible_distribution_major_version in epel repos

RHEL systems use roles/common/tasks/rhel-entitlements.yml, and this sets
Yum's $releasever to a specific RHEL minor release (eg. 8.4 or 8.6). As
a result. Fedora's MirrorManager does not return any EPEL repositories
for these minor RHEL versions.

We set a static $releasever in rhel-entitlements.yml so that we pin to
old RHEL RPM content in our old RHEL nodes. We probably need to re-think
this strategy since our CentOS Stream nodes do not (cannot) do this, and
Red Hat does not really support pinning to old versions without an EUS
subscription.

Rather than untangling all that and removing our $releasever
manipulation altogether, this commit simply hard-codes
ansible_distribution_major_version ("8", "9", etc) into the EPEL .repo
files, ignoring $releasever for EPEL.

A longer-term fix would be to stop mangling $releasever on RHEL.