This is sort of followup of MDL-75100, where we provided compatibility with strftime() and its formats, by implementing core_date::strftime() that, transparently, converts all formats between the strftime() (locale) ones and the Date/DateTime... (intl/icu) ones.

So, while everything continues working 100% the same with PHP 8.1 (where strftime() has been deprecated) and up... because of the custom core_date::strftime() implementation above... long term... we need a good plan to get rid of all those locale-based formats once and forever.

That includes:

1. Looking for all formats in codebase.
2. Looking for all formats in lang strings in all languages.
3. Converting everything to the new int/icu formats.
4. Move all uses of core_date::strftime() to use native Date/DateTime... functions, directly or using another core_date::format() new method.
5. Finally, deprecate (and remove) core_date::strftime()
6. Done, we are 100% free from locale formats forever. Party!