Allow @⁠Configuration classes to be abstract again

This commit addresses a regression introduced in 6.2.4 in conjunction
with spring-projects#34486.

See spring-projectsgh-34486
Closes spring-projectsgh-34663

Author: sbrannen

Diff for: spring-context/src/main/java/org/springframework/context/annotation/ConfigurationClassParser.java

@@ -180,8 +180,9 @@ else if (bd instanceof AbstractBeanDefinition abstractBeanDef && abstractBeanDef
 				}
 
 				// Downgrade to lite (no enhancement) in case of no instance-level @Bean methods.
-				if (!configClass.hasNonStaticBeanMethods() && ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(
-						bd.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE))) {
+				if (!configClass.getMetadata().isAbstract() && !configClass.hasNonStaticBeanMethods() &&
+						ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(
+								bd.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE))) {
 					bd.setAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE,
 							ConfigurationClassUtils.CONFIGURATION_CLASS_LITE);
 				}