`HibernateTransactionManager` compatibility with Hibernate 6.0/6.1

[odrotbohm]

We already fixed references to API removed in Hibernate 6 for HibernateJpaVendorAdapter but apparently HibernateTransactionManager also refers to the very same API.

[jhoeller]

To be clear, HibernateTransactionManager is part of our legacy orm.hibernate5 package and should be replaced with JpaTransactionManager plus HibernateJpaDialect / HibernateJpaVendorAdapter with LocalContainerEntityManagerFactoryBean since we officially only support Hibernate 6.x via JPA.

That said, it turns out that we can make HibernateTransactionManager compatible with Hibernate 6.0/6.1 with a few minor tweaks where we're calling Hibernate 5.6's internal connection handling mechanisms instead of the public API (the latter is gone in Hibernate 6.0 but the internal coordinator SPI is still the same). We won't be able to guarantee compatibility with Hibernate 6.x going forward, but for a start this allows usage of LocalSessionFactoryBean and HibernateTransactionManager for transitional purposes when upgrading from Hibernate 5.5/5.6 to 6.0/6.1.