On MySQL JavaInstantColumnType column uses DATETIME instead of TIMESTAMP

Right now, when you have a column initialized using JavaInstantColumnType (or `timestamp()` from exposed-java-time module) it will use `DATETIME` type upon table creation via SchemaUtils if MySQL database is used.

I believe `TIMESTAMP` type from MySQL would be more suitable since it conveys the same meaning as Instant class in java (aka pure time in UTC), while `DATETIME` acts more like LocalDateTime. This behaviour can be kind of misleading and recently resulted in some unexpected issues on our production server (we naturally thought that timestamp() would produce an actual TIMESTAMP and never checked)

I realise that it's probably too late to change it but this detail of implementation at least should be addressed in documentation. Some people could write their own implementations of JavaInstantColumnType which would use TIMESTAMP if they actualy need such behaviour (like we did).