Update Gitpod Dockerfile to use Java JDK 21 by default

[SohamJuneja]

Fixes JENKINS-75324. Updated dockerfile for gitpod so that it uses Java JDK 21 by default. Not tested currently because it required IntelliJ subscription to connect to Gitpod .

Testing done

Proposed changelog entries

• (skip-changelog)

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.
• 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.
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Desired reviewers

@krisstern

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).
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[krisstern]

Tested on Gitpod it worked:

[krisstern]

LGTM

[A1exKH]

LGTM.

[timja]

I've removed the changelog entry as this isn't user facing

[SohamJuneja]

@timja I've applied the desired changes.

[timja]

LGTM:

Picked up JAVA_TOOL_OPTIONS: -XX:+UseContainerSupport -XX:ActiveProcessorCount=1
Apache Maven 3.9.9 (8e8579a9e76f7d015ee5ec7bfcdc97d260186937)
Maven home: /home/gitpod/.sdkman/candidates/maven/current
Java version: 21.0.6, vendor: Eclipse Adoptium, runtime: /home/gitpod/.sdkman/candidates/java/21.0.6-tem
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "6.1.91-060191-generic", arch: "amd64", family: "unix"

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[SohamJuneja]

Thanks @krisstern @timja @A1exKH for reviewing this. Grateful to @krisstern for testing this !!