lsp-ruff: enable multi-root support

[lunik1]

Have been running this in my own configuration with no noticeable issues. The initial commit message of ruff's LSP server implementation lists multi-workspace as a supported feature.

Fixes #4831

[lunik1]

I haven't tested how this affects ruff-lsp. As it is formally deprecated, should it still be supported? Users should at least be able to revert to the old behavior with

(oset (gethash 'ruff lsp-clients) multi-root nil)

[jcs090218]

Has ruff-lsp been deprecated? 🤔 What’s the current status of the Ruff language server?

[lunik1]

There's ruff-lsp the deprecated Python wrapper around ruff, and ruff server which is built-in to ruff itself.