Disallow usage of string.To{Upper,Lower}() without explicit culture

[bdach]

Same as ppy/osu-framework#5261. Vaguely related to #18729.

I have changes planned to address the issue linked by replacing humanizer methods with game-side fixed copies but I'm PRing this separately to reduce size since this touches a good few files.

Guidelines taken when replacing:

• String is not user-facing (lookup string, filename) => convert as invariant
• String is user-facing in a context where localisation doesn't matter (tourney client, visual tests) => convert as invariant
• String is user-facing but localisation is not easy to support for whatever reason => convert as invariant for now
• String is user-facing and can be easily localised => use LocalisableString (two cases of this here).