Drop support for `root_uri` as an initialization parameter in `ruff_server` (#10743)
## Summary
Needed for https://github.com/astral-sh/ruff/pull/10686.
We no longer support `root_uri` as an initialization parameter, relying
solely on `workspace_folders` to find the working directories. This
means that the minimum supported LSP version is now `0.3.6`.
## Test Plan
When opening a folder in VS Code, you shouldn't see any errors in the
log which say `No workspace(s) were provided(...)`.