Adapt to Assert.instanceOf changes

Since SPR-15196, the behavior of `Assert` methods changed and some were
deprecated. This commit adapts to the new error mechanism implemented in
`Assert.instanceOf`.

Fixes gh-8140

spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/orm/jpa/TestEntityManagerTests.java

@@ -191,8 +191,7 @@ public class TestEntityManagerTests {
 	public void getIdForTypeWhenTypeIsWrongShouldThrowException() throws Exception {
 		TestEntity entity = new TestEntity();
 		given(this.persistenceUnitUtil.getIdentifier(entity)).willReturn(123);
-		this.thrown.expectMessage("ID mismatch Object of class [java.lang.Integer] "
-				+ "must be an instance of class java.lang.Long");
+		this.thrown.expectMessage("ID mismatch: java.lang.Integer");
 		this.testEntityManager.getId(entity, Long.class);
 	}