Introduce ClassUtils.isStaticClass() utility method

The impetus for this is to be able to use ClassUtils::isStaticClass
or the existing ClassUtils::isInnerClass as a method reference for
class-based predicates that need to differentiate between static
nested types and inner classes.

See gh-28207

spring-core/src/main/java/org/springframework/util/ClassUtils.java

@@ -831,15 +831,27 @@ public abstract class ClassUtils {
 		return javaLanguageInterfaces.contains(ifc);
 	}
 
+	/**
+	 * Determine if the supplied class is a static class.
+	 * @return {@code true} if the supplied class is a static class
+	 * @since 6.0
+	 * @see Modifier#isStatic(int)
+	 * @see #isInnerClass(Class)
+	 */
+	public static boolean isStaticClass(Class<?> clazz) {
+		return Modifier.isStatic(clazz.getModifiers());
+	}
+
 	/**
 	 * Determine if the supplied class is an <em>inner class</em>,
 	 * i.e. a non-static member of an enclosing class.
 	 * @return {@code true} if the supplied class is an inner class
 	 * @since 5.0.5
 	 * @see Class#isMemberClass()
+	 * @see #isStaticClass(Class)
 	 */
 	public static boolean isInnerClass(Class<?> clazz) {
-		return (clazz.isMemberClass() && !Modifier.isStatic(clazz.getModifiers()));
+		return (clazz.isMemberClass() && !isStaticClass(clazz));
 	}
 
 	/**