Relocate launcher classes

Create alternative launcher classes under the package
`org.springframework.boot.loader.launch` and use them in favor
of the previous location.

This update is designed to improve compatibility with future
changes in the loader.

Closes gh-37667

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-explodedApplicationClasspath.gradle

@@ -21,5 +21,5 @@ task explode(type: Sync) {
 
 task launch(type: JavaExec) {
 	classpath = files(explode)
-	mainClass = 'org.springframework.boot.loader.JarLauncher'
+	mainClass = 'org.springframework.boot.loader.launch.JarLauncher'
 }