Drop Oracle Linux 7 from CI

The Oracle Linux 7 EoL date is 31 Dec 2024, but the RHEL 7 EoL date is 30 Jun 2024, which is in six days. Given that our customers are more likely to care about RHEL than Oracle Linux, we should remove the latter from the CI, from the RPM build infrastructures, etc.

Remove Oracle Linux 7 from these places:

• BIND 9 CI
• BIND 9 images CI
• https://gitlab.isc.org/isc-private/rpms/bind/-/tree/cloudsmith
• COPR repos
• Cloudsmith repos
• identify OL7-induced workarounds and limitations in system test, images, etc.
• Edit COPR text to drop RHEL 7 instructions