Upgrade to OpenSAML 5

[snicoll]

Our build fixes the version to 4.0.1 that is the last version available from Maven Central. Spring Security has now fully moved to v5 so we need to follow suit to restore our build.

[philwebb]

For reference Spring WS fixed things in https://github.com/spring-projects/spring-ws/blob/61282b2b00a12e2d5d777170c7bd3da1a10479e6/gradle/plugins/conventions-plugin/src/main/java/org/springframework/ws/gradle/conventions/JavaBasePluginConventions.java#L50-L57