Use mainClass rather than deprecated main in BootRun examples

Closes gh-29965

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/running/boot-run-main.gradle

@@ -5,12 +5,12 @@ plugins {
 
 // tag::main[]
 bootRun {
-	main = 'com.example.ExampleApplication'
+	mainClass = 'com.example.ExampleApplication'
 }
 // end::main[]
 
 task configuredMainClass {
 	doLast {
-		println bootRun.main
+		println bootRun.mainClass
 	}
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/running/boot-run-main.gradle.kts

@@ -7,12 +7,12 @@ plugins {
 
 // tag::main[]
 tasks.getByName<BootRun>("bootRun") {
-	main = "com.example.ExampleApplication"
+	mainClass.set("com.example.ExampleApplication")
 }
 // end::main[]
 
 task("configuredMainClass") {
 	doLast {
-		println(tasks.getByName<BootRun>("bootRun").main)
+		println(tasks.getByName<BootRun>("bootRun").mainClass)
 	}
 }