[9.20] fix: usr: Don't allow statistics-channel if libxml2 and libjson-c are unsupported

When the libxml2 and libjson-c libraries are not supported, the statistics channel can't return anything useful, so it is now disabled. Use of `statistics-channel` in `named.conf` is a fatal error.

Closes #4895

Backport of MR !9423

Merge branch 'backport-4895-link-style-sheet-to-libxml2-support-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9486