Limit use of deprecated mainClassName in Gradle plugin tests

Closes gh-27320

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

@@ -0,0 +1,8 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClass = 'com.example.Application'
+}

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

@@ -4,6 +4,6 @@ plugins {
 }
 
 bootJar {
-	mainClassName = 'com.example.Application'
+	mainClass = 'com.example.Application'
 	launchScript()
 }

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

@@ -0,0 +1,8 @@
+plugins {
+	id 'war'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootWar {
+	mainClassName = 'com.example.Application'
+}