refactor: deprecate util.path.join

Work on neovim#2079.

lua/lspconfig/util.lua

@@ -107,10 +107,6 @@ M.path = (function()
     end
   end
 
-  local function path_join(...)
-    return table.concat(M.tbl_flatten { ... }, '/')
-  end
-
   -- Traverse the path calling cb along the way.
   local function traverse_parents(path, cb)
     path = vim.loop.fs_realpath(path)
@@ -165,7 +161,6 @@ M.path = (function()
   local path_separator = iswin and ';' or ':'
 
   return {
-    join = path_join,
     traverse_parents = traverse_parents,
     iterate_parents = iterate_parents,
     is_descendant = is_descendant,
@@ -363,6 +358,11 @@ function M.path.exists(filename)
   return stat and stat.type or false
 end
 
+--- @deprecated use `table.concat({"path1", "path2"})` or regular string concatenation instead
+function M.path.join(...)
+  return table.concat({ ... }, '/')
+end
+
 --- @deprecated use `vim.fs.dirname(vim.fs.find('.hg', { path = startpath, upward = true })[1])` instead
 function M.find_mercurial_ancestor(startpath)
   return vim.fs.dirname(vim.fs.find('.hg', { path = startpath, upward = true })[1])