force @timestamp to UTC

Make sure `@timestamp` is in UTC rather than whatever random timezone the app server is set to.  Eliminates the need for logstash hacks[1] to rewrite `@timestamp` when the server is using a local timezone.

Avoids the common Logstash+ElasticSearch+Kibana problem where data is stored in an index other than the one Kibana expects and searches are missing N hours of data, where N is the local offset from UTC.

[1] For example:

    filter {
    
      mutate {
        tags => ["local_ts"]
        rename => ["@timestamp", "_ts_local"]
      }
    
      date {
        tags => ["local_ts"]
        match => ["_ts_local", "ISO8601"]
        add_tag => ["fixed_local_ts"]
        remove_tag => ["local_ts"]
      }
    
      mutate {
        tags => ["fixed_local_ts"]
        remove => ["_ts_local"]
        remove_tag => ["fixed_local_ts"]
      }
    }