Set the encoding to UTF8 in the default logconfig (#4138)

changelog.d/4138.misc

@@ -0,0 +1 @@
+The default logging config will now set an explicit log file encoding of UTF-8.

synapse/config/logger.py

@@ -50,6 +50,7 @@ handlers:
         maxBytes: 104857600
         backupCount: 10
         filters: [context]
+        encoding: utf8
     console:
         class: logging.StreamHandler
         formatter: precise