Drop RHEL / CentOS / Oracle Linux 7 support
The RHEL (and clones) 7 will reach EOL in June 2024, shortly after BIND 9.20 will be released. Drop the support for building on those platforms, so we can use features of modern operating systems - newer compiler that supports at least subset of C23 and OpenSSL 1.1/3.0.
This will simplify some of the code that we are using in BIND 9.
Closes #3729 (closed)