source3/wscript: Announce deprecation of old Ceph version support

source3/wscript: Announce deprecation of old Ceph version support

*at() variants for various libcephfs APIs were added with Ceph v17.x.
Any other version less than v17.x is soon to be considered EOL[1] which
we will now indicate with the help of a warning message during configure
time. Going further such a situation will result in disabling the module
altogether with the next major Samba version after v4.20.

[1] https://docs.ceph.com/en/latest/releases/#ceph-releases-index

Signed-off-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>