[VSC] Hide Agentic Platform settings if feature is not supported on instance

Problem to solve

Customers running GitLab instances that do not support Agent Platform features (such as running on an older instance, or self-managed for instances that do not support it) can currently always see the "Duo Agent Platform" toggle within VS Code settings.

If they enable this in the IDE, the features do not work. This is a confusing situation

Proposed solution

• Hide the user-facing settings within the VS Code settings if the instance version is < 18.2.
• Do not enable Duo Agent Platform by default, experiment/beta features should always be an opt-in from the user