Align `JpaTransactionManager` default for `nestedTransactionAllowed` flag

[jhoeller]

Following up on #35212 and #35212 (comment) specifically, it turns out that JpaTransactionManager sets nestedTransactionAllowed to true despite none of the current JPA providers actually supporting nested transactions. While this might have been justified by OpenJPA's nested transaction support many years ago, it should align with Hibernate and EclipseLink these days (and also with the Hibernate-specific HibernateTransactionManager variant), leaving nestedTransactionAllowed at false by default. In that sense, the javadoc can remain as-is on 7.0 since it claims that the default is false already.