Update assertj-core to 3.21.0

Closes gh-10396

Author: rwinch

dependencies/spring-security-dependencies.gradle

@@ -46,7 +46,7 @@ dependencies {
 		api "org.apache.httpcomponents:httpclient:4.5.13"
 		api "org.aspectj:aspectjrt:$aspectjVersion"
 		api "org.aspectj:aspectjweaver:$aspectjVersion"
-		api "org.assertj:assertj-core:3.20.2"
+		api "org.assertj:assertj-core:3.21.0"
 		api "org.bouncycastle:bcpkix-jdk15on:1.69"
 		api "org.bouncycastle:bcprov-jdk15on:1.69"
 		api "org.eclipse.jetty:jetty-server:9.4.43.v20210629"