meson.build: Disallow libnfs v6 to fix the broken macOS build

meson.build: Disallow libnfs v6 to fix the broken macOS build

The macOS builds in our CI (and possibly other very recent distros)
are currently broken since the update to libnfs version 6 there.
That version apparently comes with a big API breakage. v5.0.3 was
the final release of the old API (see the libnfs commit here:
https://github.com/sahlberg/libnfs/commit/4379837 ).

Disallow version 6.x for now to get the broken CI job working
again. Once somebody had enough time to adapt our code in
block/nfs.c, we can revert this change again.

Message-ID: <20241218065157.209020-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>