Seemed overdue to finally drop support for PHP 5.x, as well as 7.x as
the 7 series has also reached end of life.
Expanded testing to include both the soon to be released 8.3 and the
next development version 8.4.
Added the ability to get the previous workday by passing in a negative
number of days along with the date to start from.
This update also includes some overhauling to allow for the library to
be more resilient to changes in the upstream API.
