Upgrade Hibernate support baseline to 5.2+

[jhoeller]

Driven by backlog requests (such as upgrading the dialects #22326) and a recent desire to expose SessionFactory and Session as the extended JPA interfaces in HibernateJpaVendorAdapter (since the older interfaces are deprecated as of Hibernate 5.2), we'll upgrade the baseline to Hibernate ORM 5.2+ for Spring Framework 5.3. This allows for cleaning up a lot of reflective code, in particular in HibernateTemplate, as well as streamlining our Hibernate/JPA resource unification (started in #21540).

Since JBoss EAP 7.2 upgraded to JPA 2.2 and Hibernate ORM 5.3 in January 2019, with even JBoss EAP 7.3 (still on Hibernate ORM 5.3.x) out in the meantime, this should also be acceptable for Spring usage on JBoss (which previously forced us to retain Hibernate ORM 5.1.x support). For Spring Boot purposes, we'll keep focusing on the Hibernate ORM 5.4.x line. For strict JPA 2.1 compatibility, the only Hibernate generation that remains fully supported by Spring is the Hibernate ORM 5.2.x line.

Note that Spring JPA setups may always choose not to use HibernateJpaVendorAdapter in which case any Hibernate version can be used. Our official baseline only applies to HibernateJpaVendorAdapter and to the native orm.hibernate5 support package; beyond that, it also indicates our support range for Spring Framework 5.3.x's extended maintenance period.