strftime() is deprecated as of PHP 8.1

[azett]

Any date format config in FlatPress relies on the strftime() format (with % escape character). As of PHP 8.1, strftime() is marked as deprecated.

Reported on the support forum by stippi.

To do: Rework all date formats in FlatPress - or at least find a solution to convert strftime() format strings into date() format strings. May be helpful:

• Convert format strings: https://gist.github.com/azett/2f786c0269798a62d7d1fdf5706fd039
• Custom strftime() function: https://gist.github.com/azett/261bbfc7075b0b9423e2a6235a29e16a

Also: Smarty 2.6.31 uses strftime() as well; need to check when there is an update. May be a good time time to switch to a more recent smarty version? [Update 2022-01-23: Yes, we need to update: #94]