ci: leap < 15.4 are dead

These packaging tests are dying anyway; the manager branch reworked them. So at least the breakages won't be shown in red until then.

https://gitlab.nic.cz/knot/knot-resolver/-/jobs/852665 https://build.opensuse.org/request/show/1050454

obs:leap15 after updating fails later in the vagrant step though: https://gitlab.nic.cz/knot/knot-resolver/-/jobs/852799