Eliminate need for reflection when calling getChangeSets

[jglick]

Downstream of jenkinsci/jenkins#2730. Inspired by jenkinsci/pipeline-model-definition-plugin#178.

@reviewbybees

[ghost]

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[abayer]

Waiting for build result. I'm not a big fan of bumping the core dependency that high, but eh, that just means that if other changes are going to go in workflow-support in the next few months, they probably need to go on a 2.14.x branch too.

[jglick]

Build is clean.

Regarding the core dep, well, you could

• merge, and create a backports branch
• put this PR on hold
• start a development branch (and I guess origin PR) for newer baseline and merge this into it

[abayer]

2.14.x branch cut, gonna merge this.

[recampbell]

@reviewbybees done