mgr/cephadm: pin haproxy image to 2.3.z

mgr/cephadm: pin haproxy image to 2.3.z

The 2.4 version fails for reasons that aren't immediate clear:

May 17 15:28:42 dael conmon[3007013]: [NOTICE]   (8) : haproxy version is 2.4.0-6cbbecf
May 17 15:28:42 dael conmon[3007013]: [NOTICE]   (8) : path to executable is /usr/local/sbin/haproxy
May 17 15:28:42 dael conmon[3007013]: [ALERT]    (8) : Cannot open configuration file/directory /var/lib/haproxy/haproxy.cfg : Permission denied

Even with mode 777 on those files, there is an error about the chroot to
/var/lib/haproxy.

For now, just stick with a working version.

Fixes: https://tracker.ceph.com/issues/50830
Signed-off-by: Sage Weil <sage@newdream.net>