Use kakaotalk_ as the puppet prefix

This is what upstream uses and also what
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1977
used.

Initially, I wanted to make the prefix more unique, in case another
Kakaotalk bridge comes along, but.. it's probably on the new bridge to
come up with a unique puppet prefix, not on us now to override upstream
decisions.

roles/matrix-bridge-appservice-kakaotalk/defaults/main.yml

@@ -117,7 +117,7 @@ matrix_appservice_kakaotalk_bridge_permissions: |
   }}
 
 matrix_appservice_kakaotalk_appservice_bot_username: kakaotalkbot
-matrix_appservice_kakaotalk_user_prefix: 'kakaotalk_as_'
+matrix_appservice_kakaotalk_user_prefix: 'kakaotalk_'
 
 # End-to-bridge encryption configuration
 matrix_appservice_kakaotalk_bridge_encryption_allow: false