Add php 8.3 support

[stronk7]

With MDL-76426 (php83 epic) near completed, it's time to ensure that pluginskel works ok with that PHP version.

This issue is a reminder about that.

Note that, #109, #116 and #121 are still open, sisters of this, about to bring php80, php81 and php82 support.

Ciao :-)