List of mod-breaking changes delayed to 2.3

Note: All pieces of codes that are supposed to be deleted or edited for the next major patch are supposed to be indicated by a comment formatted precisely as // TODO: 2.3, to make grepping easy. If you find any such code that is not marked properly, it's a good idea to add this comment.

• Remove P_PreTicker ( !2204 (closed))
  • Known to break existing mods such as Chaotix title screen, which rely on preticker garbage
• server shouldn't exist for dedicated servers
  • Would break existing mods since, in the past, server was used as a lazy version of netvars
• Lua API deprecations
• Remove global constants A to Z and a to z (use strings instead?)