Drop RHEL/CentOS 7 support after 30. June 2024

RHEL/CentOS 7 Maitentance Phase 2 ends on 30. June 2024. As BIND 9.20 will be released in Q.I 2024, there's little point in supporting RHEL/CentOS/Oracle/RockyLinux/... 7 in BIND 9.19 now. We should also drop the support for RHEL-and-clones 7 support after 30. June 2024 in BIND 9.18.

• Phase I - drop EL7 support in 9.19+
  • Drop EL7 jobs from GitLab CI for main (!7346 (merged))
  • Remove EL7 bind-dev RPM builds from Cloudsmith pipeline (https://gitlab.isc.org/isc-private/rpms/bind/-/merge_requests/19)
  • Remove EL7 buildroot from isc/bind-dev COPR
  • Update installation instructions on isc/bind-dev COPR so that they do not include EL7-specific bits
  • Remove old 9.19 EL7 packages from bind-dev Cloudsmith repo (after a few months)
• Phase II - drop EL7 support in 9.18 (after EOL - 2024-06-30)
  • Drop EL7 jobs from GitLab CI for v9_18
  • Drop EL7-specific parts from all Packer image recipes
  • Drop EL7 support from the packager:rpm Docker image
  • Drop EL7-specific parts from BIND RPM build&test scripts
  • Remove EL7 buildroots from COPR
  • Update installation instructions on COPR so that they do not include EL7-specific bits
  • Remove EL7 from CI images build scripts (Docker, Packer, ...)
  • Remove all EL7 packages from Cloudsmith (after a few months); update "Due date"