Make output from spring-boot-autoconfigure-processor reproducible

[wilkinsona]

spring-boot-autoconfigure-processor currently generates a properties file that is written with java.util.Properties.store(OutputStream, String). This has a couple of problems that prevents reproducible output:

• even with the same input, the ordering of the properties may not be consistent
• a timestamp is written in a comment at the top of the file

There is a class in Spring Framework, org.springframework.core.SortedProperties, that provides the functionality that we need but I don't think our annotation processor can safely have any dependencies. We can consider copy-paste instead.