chg: dev: fix the rsa exponent to 65537

There isn't a realistic reason to ever use e = 4294967297. Fortunately
its codepath wasn't reachable to users and can be safetly removed.

Keep in mind the `dns_key_generate` header comment was outdated. e = 3
hasn't been used since 2006 so there isn't a reason to panic. The
toggle was the public exponents between 65537 and 4294967297.

Merge branch 'aydin/rsa-cleanup' into 'main'

See merge request isc-projects/bind9!9133