Add utf8 encoding option to log config

roles/matrix-server/templates/synapse/synapse.log.config.j2

@@ -18,6 +18,7 @@ handlers:
         maxBytes: {{ matrix_synapse_max_log_file_size_mb * 1024 * 1024 }}
         backupCount: {{ matrix_synapse_max_log_files_count }}
         filters: [context]
+        encoding: utf8
     console:
         class: logging.StreamHandler
         formatter: precise