Codechange: We are heading to 15

[rubidium42]

Motivation / Problem

Once release-14 has been branched, master isn't going to become 14 any more.

Description

Essentially what #10302 did, but with all versions one higher.

Limitations

Obviously shouldn't be merged before release-14 has been branched. Technically this probably could be a 'Change' as it's eventually user visible, though it should not end up in the changelog so I've gone for 'Codechange'.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker