nvim: init: Update vim diagnostic config

The show_header option of vim.diagnostic.open_float has been renamed
to simply header and has different semantics (you can now specify a
custom header string and highlights). A value of false has the same
meaning as before.
https://github.com/neovim/neovim/pull/16328

nvim/.config/nvim/init.lua

@@ -103,7 +103,7 @@ vim.diagnostic.config({
   update_in_insert = false,
   severity_sort    = true,
   float = {
-    show_header = false,
+    header      = false,
     source      = 'always',
     border      = 'rounded',
     focusable   = false,