Require JUnit 4.9 or higher in the TCF

Prior to this commit, the Spring TestContext Framework (TCF) was
compatible with JUnit 4.5 or higher.

This commit effectively raises the minimum version of JUnit that is
officially supported by the TCF to JUnit 4.9, thereby aligning with
similar upgrades made in the Spring Framework 4.0 release (i.e.,
upgrading minimum requirements on third-party libraries to versions
released mid 2010 or later).

Issue: SPR-11908
SelectTag consistently checks specified 'multiple' attribute now, never falling back to forceMultiple in case of user-provided value

Issue: SPR-11903
Explicitly mention BeanPostProcessor declaration requirements with @Bean methods

Issue: SPR-11906
UriComponentBuilder allows for multiple independent build() calls on same builder instance

Issue: SPR-11885
ConfigurationClassPostProcessor skips pre-processed bean definitions now (avoiding side effects in case of multiple CCPPs)

Issue: SPR-11858