Bump Java version

[martinbonnin]

Closes #13082

Check list

• Related issue / work item is attached
• Tests are written (if applicable)
• Documentation is updated (if applicable)
• Changes are tested and related VM images are successfully generated

[Copilot]

Pull Request Overview

This PR updates the default Java version from 17 to 21 for Ubuntu 24.04 runner images, addressing issue #13082. The change maintains support for all existing Java versions while making Java 21 the default.

• Updated default Java version from 17 to 21 in the toolset configuration
• Updated documentation to reflect Java 21 as the new default version
• Cleaned up trailing whitespace in the documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
images/ubuntu/toolsets/toolset-2404.json	Changes default Java version from "17" to "21"
images/ubuntu/Ubuntu2404-Readme.md	Updates documentation to show Java 21 as default and removes trailing whitespace

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