Use f:dropdownDescriptorSelector for agent launcher

[jglick]

#6464 reapplied, reverting #6519.

Testing done

Fails as described (interactively) unless jenkinsci/credentials-plugin#651 is applied.

Proposed changelog entries

• Improve the display of help text for agent launch methods.

Proposed changelog category

/label rfe

Proposed upgrade guidelines

You should first make sure you have updated the credentials plugin to at least version 1447.v4cb_b_539b_5321.

Desired reviewers

@mawinter69 @jenkinsci/sig-ux

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!