Fix bootBuildImage integration tests on Java 13

We build with Java 8, 11, and 13 without specifying target
compatibility for the bytecode of the app that's placed in the image.
The built image uses Java 11 and when the app is built with Java 13
it fails to start as the Java 11 VM can't read that Java 13 bytecode.

This commit configures the app that's placed in the image to be built
with target compatibility of 1.8. This allows it to be compiled with
all version of Java with which we build and to also be compatible
with the JVM that's in the image.

See gh-19831

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

@@ -2,3 +2,6 @@ plugins {
 	id 'java'
 	id 'org.springframework.boot' version '{version}'
 }
+
+sourceCompatibility = '1.8'
+targetCompatibility = '1.8'