[JENKINS-42816] use agent terminology and fix help for label.

[aHenryJard]

See JENKINS-42816.

• use agent terminology
• the help message for label was not displayed, added the missing field value to have it displayed.

I have run tests locally, it seems ok

Screenshot of the help message (using mvn hpi:run):

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• Pull request title represents the changelog entry which will be used in Release Notes. JIRA issue is a part of the title (see existing PRs as examples)
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• Test automation, if relevant. We expect autotests for all new features or complex bugfixes
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CC

@mention

[Vlatombe]

@jtnord or @jglick would you mind merging/releasing this change?

[jglick]

https://github.com/jenkinsci/promoted-builds-plugin/releases/tag/promoted-builds-3.10