Make UnspecifiedType abstract

This should have no effect on client code, but means we can internally make it an error for an `@AutoValue` class not to be abstract. (Usually an `@AutoValue` class has abstract property methods, which means it is abstract anyway.)

PiperOrigin-RevId: 751057130

Author: eamonnmcmanus

common/src/main/java/dev/cel/common/types/UnspecifiedType.java

@@ -32,7 +32,7 @@
 @CheckReturnValue
 @Immutable
 @Internal
-public class UnspecifiedType extends CelType {
+public abstract class UnspecifiedType extends CelType {
 
   @Override
   public CelKind kind() {