Use URLClassLoader by default

[basil]

Since #5698 we have supported both URLClassLoader and AntClassLoader, with AntClassLoader as the default. The main reason to use AntClassLoader was the Bouncy Castle API plugin, but recent versions (including the version bundled in Jenkins core) have supported URLClassLoader for a while now. In addition, as of #5865 AntClassLoader had to be marked as not parallel capable, while URLClassLoader has never had such a restriction. Since it is the beginning of a new LTS cycle, I think it makes sense to switch the default to URLClassLoader, which also enables parallel class loading.

Proposed changelog entries

Use native Java Platform functionality rather than Ant to load classes. The old behavior can be restored by setting -Dhudson.ClassicPluginStrategy.useAntClassLoader=true.

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) and are in the imperative mood. 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
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• 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 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, 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 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!