Deprecate 'che-plugin-registry' and use https://open-vsx.org/

[ibuziuk]

Is your task related to a problem? Please describe

Motivation:
'che-plugin-registry' was created back in time when theia was the default editor. It also contained editor definitions that are managed as k8s resources now https://eclipse.dev/che/docs/stable/administration-guide/configuring-editors-definitions/

At this point, there is no need to support and maintain an opinionated build of the openvsx in the upstream, and it can be deprecated and archived.

Describe the solution you'd like

Upstream - use https://open-vsx.org/

• Setting up a private Open VSX registry #23148
• Extensions published on a local OpenVSX by users with privileged roles are not verified eclipse/openvsx#1025
• Enable installation of VSCode extensions from openvsx.org via the internal OpenVSX registry #23188

Downstream - build https://github.com/eclipse/openvsx from the source with the curated list of extensions

Admins struggle with the fact that the container for the custom plugin registry must be rebuilt each time there is a change. It's not an issue with a small number of extensions, but quickly gets unmanageable for a large enterprise with demand for a lot of extensions. What they would really like is something like a mini open-vsx instance that can be managed with an API, so that extensions can be added, updated, or removed dynamically.

Additional context

The first step in this direction was to decouple editor definitions from the plugin registry - #22932