sysutils/liburcu: update to 0.12.1

Since the library version changes from 6.0.0 to 6.1.0:
All ports that depend on liburcu in default configuration get
PORTREVISION bumped. All were build-tested in 12.1 amd64 poudriere bulk -t.

Reset maintainer (mmokhi@), repeated maintainer timeout.

PR: 243768
Reported by: alex_y_xu@yahoo.ca
Approved by: maintainer timeout (mmokhi, 88 d)