Enable CD for plain-credentials

[jglick]

Description

jenkinsci/bom#1336 (comment)

Reviewer checklist (not for requesters!)

• Check this if newly added person also needs to be given merge permission to the GitHub repo (please @ the people/person with their GitHub username in this issue as well). If needed, it can be done using an IRC Bot command
• Check that the $pluginId Developers team has Admin permissions while granting the access.
• In the case of plugin adoption, ensure that the Jenkins Jira default assignee is either removed or changed to the new maintainer.
• If security contacts are changed (this includes add/remove), ping the security officer (currently @Wadeck) in this pull request. If an email contact is changed, wait for approval from the security officer.

There are IRC Bot commands for it