Fixes invalid context expectation in test

[alecharp]

The workflow-step-api plugin changed the wording on the invalid context error. It has to be updated here as well.
cc @jglick

Testing done

Running the test with only the pom update and then updated the text string expectation.
This is a follow of jenkinsci/workflow-step-api-plugin#143 and coming from jenkinsci/bom#2846

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[MarkEWaite]

Thanks!

[jglick]

Better still to update

credentials-binding-plugin/src/main/java/org/jenkinsci/plugins/credentialsbinding/impl/BindingStep.java

Line 130 in 0fb18e0

throw new MissingContextVariableException(FilePath.class);

to the new constructor. I can do that as a follow-up though.

[jglick]

FWIW I have adopted the convention of putting overrides like this into dependencyManagement, mainly so that they are textually right next to the bom-* dep and thus apparent when evaluating Dependabot bumps that would allow the override to be deleted.