Update bootJar and bootWar to use new main class resolution mechanism

See gh-22922

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

@@ -0,0 +1,5 @@
+plugins {
+	id 'java'
+	id 'application'
+	id 'org.springframework.boot' version '{version}'
+}

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

@@ -0,0 +1,5 @@
+plugins {
+	id 'war'
+	id 'application'
+	id 'org.springframework.boot' version '{version}'
+}