plugins/lsp/servers: replace rootDir with rootMarkers

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.

tests/test-sources/plugins/lsp/_lsp.nix

@@ -84,12 +84,13 @@
           filetypes = [ "python" ];
           autostart = false;
         };
-        # rootDir
+        # rootMarkers
         tinymist = {
           enable = true;
-          rootDir = ''
-            require 'lspconfig.util'.root_pattern('.git', 'main.typ')
-          '';
+          rootMarkers = [
+            ".git"
+            "main.typ"
+          ];
         };
       };
     };