fix(r-language-server): use --no-echo instead of --slave (#3280)

Starting from R version 4.0.0, the command line option `--slave` were
renamed to `--no-echo`.

See the relevant news for 4.0.0:
https://cran.r-project.org/bin/windows/base/old/4.0.0/NEWS.R-4.0.0.html
and the corresponding commit:
f1ff49e745

lua/lspconfig/server_configurations/r_language_server.lua

@@ -2,7 +2,7 @@ local util = require 'lspconfig.util'
 
 return {
   default_config = {
-    cmd = { 'R', '--slave', '-e', 'languageserver::run()' },
+    cmd = { 'R', '--no-echo', '-e', 'languageserver::run()' },
     filetypes = { 'r', 'rmd' },
     root_dir = function(fname)
       return util.find_git_ancestor(fname) or vim.loop.os_homedir()