Release detached plugin class loaders on shutdown

[basil]

Similar to #6030, another minor bug noticed when running tests on Windows. PluginManager#stop releases the class loaders for active plugins, but not detached plugins (which are in the plugins list but not the activePlugins list). As such the class loaders for the detached plugins never get closed. This PR properly processes both lists.

Proposed changelog entries

• Fix a resource leak when shutting down Jenkins.

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 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 does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (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).

[basil]

This PR was tested successfully in a passing Windows CI run in #6024.

[jglick]

Took me a while to understand the diff but I think this is OK.

[basil]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process. Thanks!