Consider merging centos and fedora rulesets

[AMDmi3]

centos and fedora rulesets being separate is more of a historical artifact (centos has much less packages, so there was no need for many rules, and it came unnoticed) and it would make sense to merge them to avoid the need to duplicate rules and chose between the same things in repository configs (see repology/repology-updater#965 (comment)).

Test comparison of packages (current setup vs. one with fedora ruleset used for all centos repos) packagesets shows a few differences caused by lacking centos rules. Can start with adding them, then remove centos ruleset which would be a no-op.

Another question is whether we still need separate families for them.