Require Java 21 or newer

[uhafner]

As Java 25 is available for a couple of months now, it is time to drop support for Java 17. When this PR is merged, Jenkins will support only Java 21 and 25, or newer.

See Java support plan and mailing list discussion.

Proposed changelog entries

• Require Java 21 or newer.

Proposed changelog category

/label major-rfe

Proposed upgrade guidelines

N/A

Submitter checklist

• The 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.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, 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.
• 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, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[MarkEWaite]

Before we make progress here, I'd recommend putting an administrative monitor in place, as we did before, when we made the cut with earlier versions, and announcing a final date when we make this transition. The monitors should be present at least in one LTS line.

This helps people to raise awareness of the upcoming change. Not everyone is aware of our 2 + 2 + 2 policy, and due to the complexity of some instances, it sometimes takes months to update something.

I don't think we need an additional administrative monitor. The Java 17 administrative monitor has been telling users that Java 17 support will end March 31, 2026. I believe it is configured to start showing that warning 6 months prior to the end of life. The warning looks like this:

The LTS release would transition April 15, 2026, the release of the next LTS baseline after 2.541.x

[NotMyFault]

Before we make progress here, I'd recommend putting an administrative monitor in place, as we did before, when we made the cut with earlier versions, and announcing a final date when we make this transition. The monitors should be present at least in one LTS line.
This helps people to raise awareness of the upcoming change. Not everyone is aware of our 2 + 2 + 2 policy, and due to the complexity of some instances, it sometimes takes months to update something.

I don't think we need an additional administrative monitor. The Java 17 administrative monitor has been telling users that Java 17 support will end March 31, 2026. I believe it is configured to start showing that warning 6 months prior to the end of life. The warning looks like this:

The LTS release would transition April 15, 2026, the release of the next LTS baseline after 2.541.x

My bad! I didn't check for existing admin monitors in place. That's a good catch; we don't need another one. I've dismissed my concern.

[jglick]

🎉

Some US_USELESS_SUPPRESSION_ON_CLASS probably due to javac changes.

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!