Codechange: remove all u8 prefix in favour of compile-time option

[TrueBrain]

Motivation / Problem

Every time we try C++20, we are greeted with the issue that u8 literal is a bit .. weird, and not backwards compatible with C++17.

Now @frosch123 was like: why not set a compiler-flag and remove the u8.

As that cleans up a lot of special chars in our codebase, that sounds like an excellent plan.

Description

Remove the use of the u8 literal, and add compiler-flags to read the strings as UTF-8 strings.

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