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/
Merge pull request #978 from gbehrmann/fix/2.10/rb7402

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