HHH-9290 : Do not truncate HT_ temporary table name prefix on Oracle

`Oracle8iDialect.generateTemporaryTableName` usually prefixes all temporary table names with "HT_".  However, temporary table names for for table names longer than 27 characters just start with "T_".

This PR ensures that all temporary table names start with "HT_"

As a side-effect of this fix, temporary table name can be 30 characters long (instead of only 29).  I believe the previous 29 character limit was a mistake.

https://hibernate.atlassian.net/browse/HHH-9290