Protects against invalid hour offsets during transitions to years
following leap years.

MinGW's strftime implementation does not work and cannot be used to
determine the time zone. Besides that, the string operations are
actually unnecessary since we can get the offset from
std::localtime.

Compare localtime to gmtime to find the zone offset on all platforms.

time_zone: Remove maybe_unused

time_zone: Use s64 storages

time_zone: Catch by reference

Moves it from Settings to Common::TimeZone, since this algorithm doesn't
depend on the setting. It also lets us use it in other libraries.

common: Various fixes

time_zone: Don't double up the std::abs

Too many absolute values were causing mirrored time zones to resolve
as the same.

This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.

We can simplify this function down into a single line with the use of
fmt. A benefit with the fmt approach is that the fmt variant of
localtime is thread-safe as well, making GetOsTimeZoneOffset()
thread-safe as well.