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.
20fd0cc60b