Fix test for Java version in image building docs

See gh-22916

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/docs/PackagingDocumentationTests.java

@@ -226,7 +226,7 @@ class PackagingDocumentationTests {
 	void bootBuildImageWithCustomBuildpackJvmVersion() throws IOException {
 		BuildResult result = this.gradleBuild.script("src/docs/gradle/packaging/boot-build-image-env")
 				.build("bootBuildImageEnvironment");
-		assertThat(result.getOutput()).contains("BP_JVM_VERSION=13.0.1");
+		assertThat(result.getOutput()).contains("BP_JVM_VERSION=8.*");
 	}
 
 	@TestTemplate