Import isOverridden from the same repository

[basil]

ReflectionUtils in Stapler was meant to be a temporary private copy and almost all uses of it are being ripped out in jenkinsci/stapler#643 with the goal being to eventually rip it out completely. The canonical public API is in Util in Jenkins core, so fix up the import here.

Testing done

mvn clean verify -DskipTests

Proposed changelog entries

• N/A

Proposed changelog category

/label skip-changelog

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
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• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

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

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.
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• 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).

[MarkEWaite]

I compared Util.isOverridden from Jenkins core with ReflectionUtils.isOverridden from Stapler and found no differences in behavior. ReflectionUtils.isOverridden was added to Stapler as part of :

• [JENKINS-73258] Migrate Stapler from EE 8 to EE 9 stapler#482

It has been included in Stapler releases since:

• 1903.v994a_db_314d58 - Migrate Stapler from EE 8 to EE 9

As far as I can tell, this is a behavior preserving change. Thanks!