ceph.spec.in: use %{_sbindir} instead of ${_sbindir} in osd %preun

ceph.spec.in: use %{_sbindir} instead of ${_sbindir} in osd %preun

a37b5b5bde8c added %preun scriptlets that use ${_sbindir}, which is
shell syntax rather than an rpm macro, so it expands to empty at run
time and the scriptlet runs "/update-alternatives", failing on
uninstall/upgrade with:

  /var/tmp/rpm-tmp.K1fvm3: line 2: /update-alternatives: No such file or directory
  error: %preun(ceph-osd-crimson-2:20.3.0-5054.g33c1d671.el9.x86_64) scriptlet failed, exit status 127
  Error in PREUN scriptlet in rpm package ceph-osd-crimson.

use %{_sbindir}, like the %posttrans --install lines already do, so it
expands to /usr/sbin/update-alternatives at build time.

Fixes: https://tracker.ceph.com/issues/77323
Signed-off-by: Kefu Chai <k.chai@proxmox.com>