Add WindowsTerminal.enabled to the list of config keys for wsl-distribution.conf

[OneBlue]

Summary of the Pull Request

This will solve the pipeline issue in #13996

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR fixes a pipeline validation issue by adding the windowsterminal.enabled configuration key to the list of valid keys for /etc/wsl-distribution.conf in the distribution validation script.

Key Changes:

• Added windowsterminal.enabled to the valid configuration keys list in validate-modern.py