CORDA-4168: Fix technical debt in 5.0 release.

[chrisr3]

Backport updates from Corda Gradle plugins 6.x to the release/5.0 branch, and fix technical debt:

• Add build dependencies to the CorDapp's jar task.
• Test Cordformation with Corda 4.8.
• Publish KDocs for plugins written in Kotlin.
• Update dependencies, e.g. YAML, ClassGraph, Commons IO.