enable compatability with future Jenkins version

[jtnord]

Adapts to jenkinsci/jenkins#23873 so the tests pass on the current Jenkins version as well as a version containing the PR

discovered by jenkinsci/bom#6031 in preparation for jenkinsci/jenkins#23873

tested with mvn test -Djenkins.version=2.540-rc37747.87da_150079a_1

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[jtnord]

@jenkinsci/bitbucket-branch-source-plugin-developers could you take a look please.

[jglick]

@nfalco79 I expect this will need a release at this point. (For CD-enabled plugins we normally use the developer label.)