Deprecate Form.findMatchingInput Prototype extension in favor of a new non-Prototype replacement

[basil]

Jenkins currently extends Prototype's Form class with a Jenkins-specific Form.findMatchingInput, which currently does nothing at all when Prototype is disabled. This is used in several plugins and there is no obvious generic replacement since this is very Jenkins-specific layout. So this PR moves the logic to a new findMatchingFormInput function and rewrites it to not depend on Prototype. The Prototype Form.findMatchingInput calling convention is retained for compatibility, but consumers are encouraged to move to the new findMatchingFormInput once their core is bumped to include this PR. And to encourage consumers to do this, I have implemented the deprecated Prototype version in terms of the new non-deprecated version, logging a warning first that instructs the plugin developer about how to migrate.

Testing done

I installed the Confluence Publisher plugin which uses this code path, and I set a breakpoint on the deprecated entrypoint. I verified that the warning was logged, that the code execution went into the new implementation, and that the new implementation correctly returned a non-null value.

Proposed changelog entries

Developer: Support searches for matching form elements without the use of the Prototype JavaScript framework.

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.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• 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

Maintainer checklist

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

• 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).

[NotMyFault]

/label ready-for-merge

---

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

[ccHanibal]

Possibly causing https://issues.jenkins.io/browse/JENKINS-71383