Make required spring-web's dependence on spring-aop

Prior to this commit spring-web's dependence on spring-aop was marked
optional, however spring-web's required dependence on spring-context
transitively forces a required dependence on spring-aop anyway.

This change marks spring-aop as a required dependency of spring-web,
which a) reflects the poms created by the 3.2 Gradle build and b) helps
solve dependency issues when using higher-level Spring projects such as
Spring Security.

Issue: SPR-9573

org.springframework.web/pom.xml

@@ -134,7 +134,6 @@
       <groupId>org.springframework</groupId>
       <artifactId>spring-aop</artifactId>
       <version>${project.version}</version>
-      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>org.springframework</groupId>