Update to Spring Security 6.0.0-M6

Closes gh-836

gradle.properties

@@ -3,7 +3,7 @@ org.gradle.jvmargs=-Xmx3g -XX:+HeapDumpOnOutOfMemoryError
 org.gradle.parallel=true
 org.gradle.caching=true
 springFrameworkVersion=6.0.0-M5
-springSecurityVersion=5.7.1
+springSecurityVersion=6.0.0-M6
 springJavaformatVersion=0.0.31
 springJavaformatExcludePackages=org/springframework/security/config org/springframework/security/oauth2
 checkstyleToolVersion=8.34