CONTRIB: move some admin-related sub-projects to admin/

The following components were moved to admin/ because they're generally
used in field by admins:

  iprange/  netsnmp-perl/  selinux/  systemd/  wireshark-dissectors/
  syntax-highlight/ release-estimator/
diff --git a/MAINTAINERS b/MAINTAINERS
index 1e9c29f..2497a36 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -95,7 +95,7 @@
 
 Master-worker
 Maintainer: William Lallemand <wlallemand@haproxy.com>
-Note: Please CC William when touching src/haproxy.c and contrib/systemd
+Note: Please CC William when touching src/haproxy.c and admin/systemd
 
 Multi-threading
 Maintainer: Christopher Faulet <cfaulet@haproxy.com>