Remove WebSocket support for Jetty 9

[jglick]

Cleans up after #6801. Retaining support temporarily for Jetty 9 seems like a sensible conservative decision but I do not think we wish to maintain this code indefinitely.

Proposed changelog entries

• Remove support for WebSocket agents when running inside Jetty 9.

Proposed upgrade guidelines

If you are using WebSocket agents and were manually embedding Jenkins in a Jetty 9 servlet container, you should either upgrade the container to Jetty 10, or simply use the built-in container (java -jar jenkins.war).

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]

Seems a bit aggressive to me so long as current versions of JenkinsRule are still based on Jetty 9. org.csanchez.jenkins.plugins.kubernetes.pipeline.WebSocketTest seems likely to break.

[jglick]

True, this should wait for jenkinsci/jenkins-test-harness#453.

[basil]

True, this should wait for jenkinsci/jenkins-test-harness#453.

I concur, and that in turn needs to wait for the test harness to require Java 11, which in turn needs to wait for the test harness to require a baseline of 2.357 or later. Since that baseline just came out in LTS form this month, I think it is a bit aggressive to start requiring it from the plugin POM and test infrastructure, but in a few more months (when more plugins have moved their baseline to 2.361.1 or later) the time could be right.

[jglick]

Since jenkinsci/jenkins-test-harness#453 has long since been widely adopted, I believe this is now safe from a toolchain perspective.

[basil]

Are all the plugins in BOM using a sufficiently new test harness with jenkinsci/jenkins-test-harness#453? If not, this might have to wait until jenkinsci/plugin-compat-tester#489.

[jglick]

I suspect it suffices for all plugins which test WebSocket agents to use the new harness. Can check an incremental build.

[jglick]

Indeed, bom passes against this change.

[NotMyFault]

Thanks!

[NotMyFault]

/label ready-for-merge

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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!