runtime(vim): update Vim Syntax generator

- Add missing "Last Change:" line.
- The date on this line in vim.vim is updated by update_date.vim at
  Make time. (I made a mistake in the file path)
- Remove unnecessary "b:loaded_syntax_vim_ex".
- Remove "Base File Date:" line in vim.vim.base
- Add Doug Kearns as Maintainer

closes: #14031

Signed-off-by: h-east <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/generator/update_date.vim

@@ -2,7 +2,7 @@
 "     '" Last Change:  '
 "
 language C
-silent new vim.vim
+silent new ../vim.vim
 normal gg
 let pat = '^"\s*Last\s*Change:\s\+'
 let lnum = search(pat, 'We', 10)