Polishing.

Add missing `@FunctionalInterface` to Specification interfaces.
`SpecificationUnitTests` shouldn't be `Serializable`.

Closes #3452

Author: quaff

spring-data-jpa/src/main/java/org/springframework/data/jpa/domain/Specification.java

@@ -39,6 +39,7 @@
  * @author Daniel Shuy
  * @author Sergey Rukin
  */
+@FunctionalInterface
 public interface Specification<T> extends Serializable {
 
 	@Serial long serialVersionUID = 1L;

spring-data-jpa/src/test/java/org/springframework/data/jpa/domain/SpecificationUnitTests.java

@@ -46,10 +46,9 @@
  * @author Mark Paluch
  * @author Daniel Shuy
  */
-@SuppressWarnings("serial")
 @ExtendWith(MockitoExtension.class)
 @MockitoSettings(strictness = Strictness.LENIENT)
-class SpecificationUnitTests implements Serializable {
+class SpecificationUnitTests {
 
 	private Specification<Object> spec;
 	@Mock(serializable = true) Root<Object> root;
@@ -163,7 +162,7 @@ void specificationsShouldBeSerializable() {
 
 		assertThat(specification).isNotNull();
 
-		@SuppressWarnings("unchecked")
+		@SuppressWarnings({"unchecked", "deprecation"})
 		Specification<Object> transferredSpecification = (Specification<Object>) deserialize(serialize(specification));
 
 		assertThat(transferredSpecification).isNotNull();
@@ -178,7 +177,7 @@ void complexSpecificationsShouldBeSerializable() {
 
 		assertThat(specification).isNotNull();
 
-		@SuppressWarnings("unchecked")
+		@SuppressWarnings({"unchecked", "deprecation"})
 		Specification<Object> transferredSpecification = (Specification<Object>) deserialize(serialize(specification));
 
 		assertThat(transferredSpecification).isNotNull();