fix upper modifier and strftime usage in date_format if PHP > 8.1

[cmelius]

This fixes:
"|upper" and
"|date_format" (see #750 )
on PHP8.1

function php8strftime should probably be moved to some function.strftime.php to use with other occurrences of strftime?

[wisskid]

@cmelius yes, the strftime-fix is too specific. strftime is also used in libs/plugins/function.html_select_date.php. See #672.
The upper changes seem fine. Do you want to update your PR? Or shall I copy your changes for the upper modifier?

[cmelius]

You're welcome to copy. I am on vacation for the next two weeks so please go ahead.

Cheers

[wisskid]

will do, closing this.