Fail with Gradle bootBuildImage and war packaging

Prior to this commit, running the bootBuildImage Gradle task on a
project configured for war packaging would result in a jar file being
built and used in the image instead of the war file. With this commit
an error will be thrown from the plugin in this case.

Fixes gh-24521

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

@@ -0,0 +1,11 @@
+plugins {
+	id 'war'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootBuildImage {
+	jar = bootWar.archiveFile
+}
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'

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

@@ -3,5 +3,9 @@ plugins {
 	id 'org.springframework.boot' version '{version}'
 }
 
+if (project.hasProperty('applyWarPlugin')) {
+	apply plugin: 'war'
+}
+
 sourceCompatibility = '1.8'
 targetCompatibility = '1.8'