SpringBoot Retain distinction between compile and runtime deps of optional deps

A recent change in JUnit 5.8 has revealed a limitation of our optional dependencies plugin. It was first noticed in Spring Framework which maintains its own copy of the plugin. We should make a similar change to our version of the plugin, both to avoid problems with a JUnit 5.8 upgrade in 2.6 and also to stop leaking the runtime dependencies of optional dependencies onto the compile classpath.