Change @Bean method signature to return RsaKeyConversionServicePostProcessor instead of BeanFactoryPostProcessor

It's friendly for Spring Boot's `@ConditionalOnMissingBean`, and:

>> When defining a Spring `@Bean` method, it is generally recommended to declare the most specific type possible as the method's return type. This means returning the concrete class of the bean, or the most specific interface that the bean implements and through which it will be referenced in the application.

Signed-off-by: Yanming Zhou <zhouyanming@gmail.com>

config/src/main/java/org/springframework/security/config/annotation/web/configuration/WebSecurityConfiguration.java

@@ -33,7 +33,6 @@ import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.config.BeanDefinition;
-import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.support.BeanDefinitionBuilder;
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
@@ -79,6 +78,7 @@ import org.springframework.web.filter.ServletRequestPathFilter;
  *
  * @author Rob Winch
  * @author Keesun Baik
+ * @author Yanming Zhou
  * @since 3.2
  * @see EnableWebSecurity
  * @see WebSecurity
@@ -190,7 +190,7 @@ public class WebSecurityConfiguration implements ImportAware {
 	}
 
 	@Bean
-	public static BeanFactoryPostProcessor conversionServicePostProcessor() {
+	public static RsaKeyConversionServicePostProcessor conversionServicePostProcessor() {
 		return new RsaKeyConversionServicePostProcessor();
 	}
 

config/src/main/java/org/springframework/security/config/annotation/web/reactive/WebFluxSecurityConfiguration.java

@@ -21,7 +21,6 @@ import java.util.List;
 
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -44,6 +43,7 @@ import static org.springframework.security.config.Customizer.withDefaults;
 
 /**
  * @author Rob Winch
+ * @author Yanming Zhou
  * @since 5.0
  */
 @Configuration(proxyBeanMethods = false)
@@ -100,7 +100,7 @@ class WebFluxSecurityConfiguration {
 	}
 
 	@Bean
-	static BeanFactoryPostProcessor conversionServicePostProcessor() {
+	static RsaKeyConversionServicePostProcessor conversionServicePostProcessor() {
 		return new RsaKeyConversionServicePostProcessor();
 	}