Tests fail on PHP 8.1

[osma]

At which URL did you encounter the problem?

Any recent CI jobs, e.g. https://github.com/NatLibFi/Skosmos/runs/5185995020?check_suite_focus=true

What steps will reproduce the problem?

1. Try to run unit tests on PHP 8.1

What is the expected output? What do you see instead?

Expected: Tests should pass.
Reality: "There were 474 errors"

The main culprit seems to be EasyRdf, which isn't yet compatible with PHP 8.1, but there's an open PR which should hopefully fix this before long.

[osma]

EasyRdf development seems to have stalled (again), but there is a more up to date fork available which is compatible with PHP 8.1: https://github.com/sweetrdf/easyrdf
We could consider switching to that.