fix: flatten backwards compatibility

Apply code review suggestion for backwards compatibility.

Co-authored-by: Lukas Reineke <lukas@reineke.jp>

lua/virt-column/utils.lua

@@ -13,7 +13,8 @@ end
 ---@vararg T
 ---@return T
 M.tbl_join = function(...)
-    return vim.iter({ ... }):flatten():totable()
+    ---@diagnostic disable-next-line: deprecated
+    return vim.iter and vim.iter({ ... }):flatten():totable() or vim.tbl_flatten { ... }
 end
 
 ---@generic T