Remove Gitpod support

Summary

Gitpod Classic will be sunset on October 15th, 2025. After this date, it's no longer supported to use Gitpod (or its successor Ona) with GDK to contribute to GitLab.

See: https://ona.com/stories/gitpod-classic-payg-sunset

Proposal

Remove all Gitpod-related configuration, documentation, and infrastructure from the GDK repository.

Timeline

This work should be completed after October 15th, 2025, when Gitpod Classic is officially sunset.