API Remove PHPUnit 5.7 compatability hacks

[GuySartorelli]

Removes dual-class implementations from #10028
Removes dual-support functionality from #10150
Also remove unnecessary conflicts in composer.json

Parent issue

• Remove support for PHPUnit 5.7 #10398

[sabina-talipova]

@GuySartorelli , there are few small questions.
Should we use phpdocs and typehints together? Could you check, please, there are few places where you have both type docs.
And there is one failed CI, but it looks like failed staff aren't related to your changes.
If everything is Ok, I'll merge.

[GuySartorelli]

The failing prefer-lowest test is also failing on the 5 branch - it's unrelated to this PR so you can disregard it.
I've updated the phpdocs.