Remove mentions of JUnit 4

This commit removes all mentions of JUnit 4 from the Javadoc and
reference documentation. It also harmonizes to simply refers to JUnit.

Closes gh-47228

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/features/kotlin.adoc

@@ -142,8 +142,7 @@ Note that some features (such as detecting the default value or deprecated items
 [[features.kotlin.testing]]
 == Testing
 
-While it is possible to use JUnit 4 to test Kotlin code, JUnit 5 is provided by default and is recommended.
-JUnit 5 enables a test class to be instantiated once and reused for all of the class's tests.
+JUnit enables a test class to be instantiated once and reused for all of the class's tests.
 This makes it possible to use javadoc:org.junit.jupiter.api.BeforeAll[format=annotation] and javadoc:org.junit.jupiter.api.AfterAll[format=annotation] annotations on non-static methods, which is a good fit for Kotlin.
 
 To mock Kotlin classes, https://mockk.io/[MockK] is recommended.

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/index.adoc

@@ -8,7 +8,7 @@ Most developers use the `spring-boot-starter-test` starter, which imports both S
 
 [TIP]
 ====
-If you have tests that use JUnit 4, JUnit 5's vintage engine can be used to run them.
+If you have tests that still use JUnit 4, JUnit's vintage engine can be used to run them.
 To use the vintage engine, add a dependency on `junit-vintage-engine`, as shown in the following example:
 
 [source,xml]

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/spring-boot-applications.adoc

@@ -9,8 +9,7 @@ Spring Boot provides a javadoc:org.springframework.boot.test.context.SpringBootT
 The annotation works by xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.detecting-configuration[creating the javadoc:org.springframework.context.ApplicationContext[] used in your tests through javadoc:org.springframework.boot.SpringApplication[]].
 In addition to javadoc:org.springframework.boot.test.context.SpringBootTest[format=annotation] a number of other annotations are also provided for xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.autoconfigured-tests[testing more specific slices] of an application.
 
-TIP: If you are using JUnit 4, do not forget to also add `@RunWith(SpringRunner.class)` to your test, otherwise the annotations will be ignored.
-If you are using JUnit 5, there is no need to add the equivalent `@ExtendWith(SpringExtension.class)` as javadoc:org.springframework.boot.test.context.SpringBootTest[format=annotation] and the other `@...Test` annotations are already annotated with it.
+If you are using JUnit, there is no need to add the equivalent `@ExtendWith(SpringExtension.class)` as javadoc:org.springframework.boot.test.context.SpringBootTest[format=annotation] and the other `@...Test` annotations are already annotated with it.
 
 By default, javadoc:org.springframework.boot.test.context.SpringBootTest[format=annotation] will not start a server.
 You can use the `webEnvironment` attribute of javadoc:org.springframework.boot.test.context.SpringBootTest[format=annotation] to further refine how your tests run:

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/test-scope-dependencies.adoc

@@ -3,7 +3,7 @@
 
 The `spring-boot-starter-test` starter (in the `test` `scope`) contains the following provided libraries:
 
-* https://junit.org/junit5/[JUnit 5]: The de-facto standard for unit testing Java applications.
+* https://junit.org/junit5/[JUnit]: The de-facto standard for unit testing Java applications.
 * {url-spring-framework-docs}/testing/integration.html[Spring Test] & Spring Boot Test: Utilities and integration test support for Spring Boot applications.
 * https://assertj.github.io/doc/[AssertJ]: A fluent assertion library.
 * https://github.com/hamcrest/JavaHamcrest[Hamcrest]: A library of matcher objects (also known as constraints or predicates).