Remove mentions of JUnit 4 from javadoc and reference documentation

[wilkinsona]

Spring Framework 7 has deprecated support for JUnit 4 and will remove it in 7.1 or 7.2. Aligned with this, we should consider what we want to do and when. For example, our javadoc mentions JUnit 4 in a number of places such as:

spring-boot/module/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/orm/jpa/DataJpaTest.java

Lines 66 to 67 in 96afc3f

	* When using JUnit 4, this annotation should be used in combination with
	* {@code @RunWith(SpringRunner.class)}.

This is common across many (all?) of our test slice annotations. Should we remove this and similar now or wait until support's completely removed from Framework?

[snicoll]

See also #47256

[wilkinsona]

I thought we'd decided to leave this in place for 4.0 (hence the milestone remaining 4.x). #47256 was created for the things that we wanted to do in 4.0.

[snicoll]

Sorry, I think I have a recollection of that now that you mention it.