MDL-71622 behat: use own fork of friends-of-behat/mink-extension

MDL-71622 behat: use own fork of friends-of-behat/mink-extension

Upstream behat/mink-extension isn't updated since 2018.

So we switch to friends-of-behat/mink-extension until
upstream situation is clarified.

But friends-of-behat/mink-extension has some PHP8 issues
that, at the time of writting this, have not been solved:

- https://github.com/FriendsOfBehat/MinkExtension/pull/9

So we are temporarily changing to one fork:

https://github.com/moodlehq/MinkExtension

Whenever those issues are solved by friends-of-beha or behat
we will change back to them and remove our fork.

Generated with composer 2.x and PHP 7.3