Remove usage of gipfl\Format\LocalTimeFormat in favor of direct use of intl also fixing bugs

[lippserd]

When using intl formatters, we may pass C as the locale which is not a valid ICU locale (note that this is not just gipfl specific). This was fine until a recent change in PHP, which now throws an exception in PHP >= 8.1.25. Since patching gipfl is not an option, we will simply replace it with using intl directly. Note that it is important not to use setlocale(..., 0) to pass the locale to intl, as this could pass C, which is invalid. I suggest just using null as this will automatically retrieve the default/set ICU locale, or Locale::getDefault() which does the same.

It also makes sense to replace strtotime() calls with new DateTime() where the code will change.

Upon creating a PR, make sure to also reference (resolve) the following issues:

• Error on Activity Log #2828
• datefmt_create: invalid locale: U_ILLEGAL_ARGUMENT_ERROR (LocalDateFormat.php:27) icingaweb2-module-incubator#11