use .NET 6 - msbuild update broke omnisharp on legacy build

vim/vimfiles/vimrc

@@ -385,6 +385,7 @@ let g:ale_set_loclist = 1
 " }}}
 
 " OmniSharp: {{{
+let g:OmniSharp_server_use_net6 = 1
 let g:OmniSharp_popup_position = 'peek'
 if has('nvim')
   let g:OmniSharp_popup_options = {