Add missing matrix_bot_maubot_admins variable to defaults for matrix-bot-maubot

roles/custom/matrix-bot-maubot/defaults/main.yml

@@ -58,6 +58,8 @@ matrix_bot_maubot_server_port: 29316
 
 matrix_bot_maubot_unshared_secret: 'generate'
 
+matrix_bot_maubot_admins: []
+
 # Specifies the default log level for all bot loggers.
 matrix_bot_maubot_logging_level: WARNING