Remove: officially mark Vista as no longer supported

[TrueBrain]

Motivation / Problem

#11529 reports Vista is no longer working. Vista has been EoL for over 11 years. And as it goes, we tend to not deliberately break old OSes, but if they no longer work (as we updated tools, MSVC no longer targets it, etc etc) we drop support for it to.

Closes #11529.

Description

It is very likely Vista hasn't been working for years, but the amount of users that use an OS that has been EoL for over 11 years is very small, so reports happen rarely.

Limitations

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

[PeterN]

Vista was rubbish anyway ;)