Rationalize JUnit5 dependency management

This commit only provides dependency management for JUnit Jupiter,
excluding experimental and migration support artifacts. It also removes
dependency management for the platform as it is used by IDEs and build
tools). The `junit-platform.version` is still available though to allow
users to configure their plugin with a supported placeholder for the
version.

Also, given that `junit-vintage-engine`'s version does not match the
JUnit 4 version it supports, we will not add dependency management for
it.

Closes gh-10240

Author: snicoll

spring-boot-dependencies/pom.xml

@@ -948,16 +948,6 @@
 				<artifactId>junit-jupiter-engine</artifactId>
 				<version>${junit-jupiter.version}</version>
 			</dependency>
-			<dependency>
-				<groupId>org.junit.platform</groupId>
-				<artifactId>junit-platform-engine</artifactId>
-				<version>${junit-platform.version}</version>
-			</dependency>
-			<dependency>
-				<groupId>org.junit.platform</groupId>
-				<artifactId>junit-platform-launcher</artifactId>
-				<version>${junit-platform.version}</version>
-			</dependency>
 			<dependency>
 				<groupId>mysql</groupId>
 				<artifactId>mysql-connector-java</artifactId>

spring-boot-samples/spring-boot-sample-junit-jupiter/pom.xml

@@ -38,11 +38,6 @@
 				</exclusion>
 			</exclusions>
 		</dependency>
-		<dependency>
-			<groupId>org.junit.platform</groupId>
-			<artifactId>junit-platform-launcher</artifactId>
-			<scope>test</scope>
-		</dependency>
 		<dependency>
 			<groupId>org.junit.jupiter</groupId>
 			<artifactId>junit-jupiter-api</artifactId>