`spring.application.exclude` is retrieved via the
`RelaxedPropertyResolver` API explicitly and it does not have any
standard API to retrieve a list of values. As a consequence that property
could only be specified as a comma-separated value.

This felt convoluted in YAML. `RelaxedPropertyResolver` has now a
`getProperties` method that works with both comma-separated value and
index elements (i.e. list).

Closes gh-4352