Support for ReactPHP Promise v3, avoid unhandled promise rejections

[SimonFrings]

The test suite is currently reporting a lot of unhandled promise rejections with Promise v3 in this project. We originally created forward compatibility with Promise v3 in #106 back in 2022, but there were some additional changes added to the v3 release in 2023, which couldn't be covered by the time (reactphp/promise#248 and others). The composer.json was updated to confirm Promise v3 compatibility in #106, which is currently not the case.

I looked into this for several hours now and I haven't really made much progress with this yet. Avoiding the unhandled promise rejections seems to be way more complicated in here compared to all our other projects. The ReactPHP v3 release is not far away so we'll probably revisit this not too far in the future, but I can't give an exact date when we'll have the proper time to take a look at this.

This is currently blocking a lot of other topics (#112, #113, #114) and will probably block any upcoming ones as long as we're installing Promise v3. I suggest we go back to use the current Promise v2 for now until this is resolved. Input is much appreciated :)

If someone needs this (more) urgently, there's always the possibility to reach out to us or help us prioritizing by becoming a sponsor ❤️