nvim-lspconfig historically used `root_dir`, along with util functions
like `root_pattern`.
Now that neovim's own LSP API is used, `root_dir` appears to be subtly
different and `root_markers` is introduced to replace `util.root_pattern`.
Since we cannot easily warn about the `root_dir` differences, it can
usually be replaced with `root_markers`, and can still be manually
configured via `extraOptions` if needed; the simplest approach here is
to remove the `rootDir` option.
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