Updated DB client timezone to Z

- whilst `UTC` was working, it's technically not supported as per the
  docs for `mysql`: https://www.npmjs.com/package/mysql#connection-options
- `mysql2`, however, is a bit more strict and throws a warning for
  unsupported values, so it was flagging up when I was looking at
  switching to that library
- this commit switches over to `Z` AKA Zulu -
  https://en.wikipedia.org/wiki/Coordinated_Universal_Time so both
  libraries are happy

core/server/data/db/connection.js

@@ -33,7 +33,7 @@ function configure(dbConfig) {
     }
 
     if (client === 'mysql') {
-        dbConfig.connection.timezone = 'UTC';
+        dbConfig.connection.timezone = 'Z';
         dbConfig.connection.charset = 'utf8mb4';
 
         // NOTE: disabled so that worker processes can use the db without