Update to thymeleaf-extras-springsecurity6

Closes gh-837

docs/src/docs/asciidoc/examples/spring-authorization-server-docs-examples.gradle

@@ -19,7 +19,7 @@ dependencies {
 	implementation "org.springframework.boot:spring-boot-starter-oauth2-client"
 	implementation "org.springframework.boot:spring-boot-starter-oauth2-resource-server"
 	implementation "org.springframework.boot:spring-boot-starter-data-jpa"
-	implementation "org.thymeleaf.extras:thymeleaf-extras-springsecurity5"
+	implementation "org.thymeleaf.extras:thymeleaf-extras-springsecurity6"
 	implementation project(":spring-security-oauth2-authorization-server")
 	runtimeOnly "com.h2database:h2"
 	testImplementation "org.springframework.boot:spring-boot-starter-test"

samples/messages-client/samples-messages-client.gradle

@@ -20,7 +20,7 @@ dependencies {
 	implementation "org.springframework.boot:spring-boot-starter-oauth2-client"
 	implementation "org.springframework:spring-webflux"
 	implementation "io.projectreactor.netty:reactor-netty"
-	implementation "org.thymeleaf.extras:thymeleaf-extras-springsecurity5"
+	implementation "org.thymeleaf.extras:thymeleaf-extras-springsecurity6"
 	implementation "org.webjars:webjars-locator-core"
 	implementation "org.webjars:bootstrap:3.4.1"
 	implementation "org.webjars:jquery:3.4.1"