Fixes #54772

When "Workspace: close project" was used in a multi-project window, the
handler pre-created an empty "get started" workspace and activated it
before removing the old project group. This prevented
`remove_project_group` from detecting that the active workspace was
being removed, so the neighbor-fallback never ran — leaving the user on
the empty workspace even though other projects were still open.

The fix delegates entirely to `remove_project_group`, which already
handles:
- Computing a neighboring project group
- Prompting for unsaved changes via `prepare_to_close`
- Activating the neighbor (or creating an empty workspace if none
remains)

This matches the behavior of the sidebar "Remove Project" action, which
uses the same `remove_project_group` path.

Release Notes:

- Fixed "Workspace: close project" action not switching to another
project in multi-project workspace

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Co-authored-by: Eric Holk <eric@zed.dev>