The layer getMetadata response now includes all supported styles

I looked if there would be a nice way to get these programmatically but it wasn't straightforward.

For reference this value is hard-coded in the other places it is used. e.g. https://github.com/aodn/ncWMS/blob/63691070a81bdfee3fd0721bccb309e1086fb785/web/WEB-INF/jsp/capabilities_xml.jsp#L134
This is the same order they appear in the GetCapabilities document (cut and paste FTW). It doesn't really matter to us as we sort them in the Portal anyway.