Move telegraf_output from global pillar to telegraf pillar

The Telegraf backend selector lived at global.telegraf_output but it is
a Telegraf-scoped setting, not a cross-cutting grid global. Move both
the value and the UI annotation under the telegraf pillar so it shows
up alongside the other Telegraf tuning knobs in the Configuration UI.

- salt/telegraf/defaults.yaml:    add telegraf.output: BOTH
- salt/telegraf/soc_telegraf.yaml: add telegraf.output annotation
- salt/global/defaults.yaml:      remove global.telegraf_output
- salt/global/soc_global.yaml:    remove global.telegraf_output annotation
- salt/vars/globals.map.jinja:    drop telegraf_output from GLOBALS
- salt/firewall/map.jinja:        read via pillar.get('telegraf:output')
- salt/postgres/telegraf_users.sls: read via pillar.get('telegraf:output')
- salt/telegraf/etc/telegraf.conf: read via TELEGRAFMERGED.output
- salt/postgres/tools/sbin/so-stats-show: update user-facing docs

No behavioral change — default stays BOTH.

salt/global/defaults.yaml

@@ -1,4 +1,3 @@
 global:
   pcapengine: SURICATA
-  pipeline: REDIS
-  telegraf_output: BOTH
+  pipeline: REDIS

salt/global/soc_global.yaml

@@ -59,13 +59,5 @@ global:
     description: Allows use of Endgame with Security Onion. This feature requires a license from Endgame.
     global: True
     advanced: True
-  telegraf_output:
-    description: Selects the backend(s) Telegraf writes metrics to. INFLUXDB keeps the current behavior; POSTGRES writes to the grid's Postgres instance; BOTH dual-writes for migration validation.
-    options:
-      - INFLUXDB
-      - POSTGRES
-      - BOTH
-    global: True
-    advanced: True
     helpLink: influxdb