Update dependency org.springdoc:springdoc-openapi-starter-webmvc-ui to v2.8.4

Author: renovate[bot]

Diff for: build.gradle

@@ -276,7 +276,7 @@ dependencies {
   implementation group: 'com.github.hmcts', name: 'ccd-case-document-am-client', version: '1.7.3'
 
   implementation group: 'org.apache.tomcat.embed', name: 'tomcat-embed-core'
-  implementation group: 'org.springdoc', name: 'springdoc-openapi-starter-webmvc-ui', version: '2.6.0'
+  implementation group: 'org.springdoc', name: 'springdoc-openapi-starter-webmvc-ui', version: '2.8.4'
   implementation group: 'org.springframework.boot', name: 'spring-boot-starter-web'
   implementation group: 'org.springframework.boot', name: 'spring-boot-starter-actuator'
   implementation group: 'org.springframework.boot', name: 'spring-boot-starter-aop'