Update "Extensions Marketplace" admin settings in dev, staging, and .com

MR: Pending...

Description

Before we update the Web IDE to use the Extensions Marketplace admin settings, we need to preset the admin settings in the .com, staging, and dev environments to avoid any disruption to the feature.

Acceptance Criteria

• Enable feature flag vscode_extensions_marketplace_settings on .com, staging, and dev. DO NOT enable by default. Follow the steps [Feature flag] Rollout of `vscode_extension_mar... (#508996 - closed).
• Have a GitLab administrator for .com, staging, and dev update the admin UI to use the Open VSX preset for Extensions Marketplace under Admin > Settings > General. Follow the Change Request workflow laid out in the handbook.

NOTE: This issue likely does not require an MR. It's a placeholder for an admin task.