fix(extras): underline = false for setting diagnostics (#3379)

Similar to https://github.com/folke/lazy.nvim/commit/ea7b9c3c3fd9026e1a5ae27950585df9a42ccd5b#
I only noticed this, because in my main config I have some Extras not
managed by `LazyExtras`.

lua/lazyvim/util/extras.lua

@@ -200,7 +200,7 @@ function X:update()
       diag.lnum = diag.row - 1
       return diag
     end, self.diag),
-    { signs = false, virtual_text = true }
+    { signs = false, virtual_text = true, underline = false }
   )
 end