springboot spring.config.additional-location is ignored when spring.config.location is configured

This issue relates to #20743.

When we fixed https://github.com/spring-projects/spring-boot/issues/10595, I don't think we considered a scenario where both spring.config.additional-location and spring.config.location are configured. As things stand, when spring.config.location is configured, spring.config.additional-location is ignored. I'm not sure that this is the right behaviour.

The documentation for spring.config.location states that its locations "replace the default locations". The documentation for spring.config.addition-location states that its locations "are used in addition to the default locations". Given this description of the behaviour, I think it's reasonable to expect the additional locations to be used as well as those that have replaced the defaults.