srm: Mark srm.enable.space-reservation.implicit forbidden

When upgrading, sites that used to enable implicit space reservation in SRM
must enable the equivalent option in space manager. To force this change, this
patch marks the old properties as forbidden.

Target: 2.10
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7402/
(cherry picked from commit ec26c94df25ff352763b34a816dddb5750677b4b)
Merge pull request #979 from gbehrmann/fix/2.9/rb7402

srm: Mark srm.enable.space-reservation.implicit forbidden