collab: Make unsupported for MinGW toolchain (#23518)

Closes #23451

reverts #23117 for MinGW. collab can't be compiled for MinGW because
webrtc itself doesn't support MinGW compilers

Release Notes:

- N/A

docs/src/development/local-collaboration.md

@@ -6,6 +6,8 @@ First, make sure you've installed Zed's backend dependencies for your platform:
 - [Linux](./linux.md#backend-dependencies)
 - [Windows](./windows.md#backend-dependencies)
 
+Note that `collab` can be compiled only with MSVC toolchain on Windows
+
 ## Database setup
 
 Before you can run the `collab` server locally, you'll need to set up a `zed` Postgres database.