Fix ABI break by moving PROCSIG_SLOTSYNC_MESSAGE in ProcSignalReason

Fix ABI break by moving PROCSIG_SLOTSYNC_MESSAGE in ProcSignalReason

Commit 15910b1c363 PROCSIG_SLOTSYNC_MESSAGE in the middle of
enum ProcSignalReason, breaking the ABI.

Fix this by moving PROCSIG_SLOTSYNC_MESSAGE to just before the last entry,
NUM_PROCSIGNALS, to restore ordering of other entries.

This increases the value of NUM_PROCSIGNALS, which technically changes the ABI.
However, since it represents the number of enum entries (not a signal reason),
and no affected third-party code is known, this change will be recorded in
.abi-compliance-history later.

Per buildfarm member crake.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwH_AAbtsiYDJt65N7_4PJ0CgOJmBEaCq68e5_tcuG_vXw@mail.gmail.com
Backpatch-through: 17 only