Use older Piston version for Java 21 compatibility

[SirYwell]

Overview

Description

To still support Java 21, we need to use an older Piston version.
From a quick look, it doesn't seem like we're missing out on anything important.

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.

[Copilot]

Pull request overview

This PR updates the Gradle version catalog to keep the project compatible with Java 21 by downgrading the EngineHub Piston dependency to a version that still supports Java 21 at runtime.

Changes:

• Downgrade piston from 0.6.0 to 0.5.11 in the version catalog.

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[MattBDev]

While I'm okay with downgrading for now. I hope we can put a timeline on moving to Java 25.

[PierreSchwang]

Could potentially add to dependabot exclusion-list

[Copilot]

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