Allow nikic/php-parser v5 to be used once we move to php80 as requirement

[stronk7]

It seems that both phpunit and php-code-coverage have started to accept nikic/php-parser v5, but that's leading to problems with PHP 7.4 runs, that we still support.

So, with this commit, we locked nikic/php-parser v4: https://github.com/moodlehq/moodle-cs/pull/89/commits/6eb72537f80a0f333472123ad172909d55c0410d in January 2024.

Once we officially make moodle-cs to only support php >= 8.0 we can remove this lock (basically, remove the commit above).

This issue will be in charge of that.