Adjust composer files for 'drop PHP 5.6'

[phil-davis]

Description

I adjusted the behat/composer.json I did not adjust info.xml min-version, because we only have a "latest" testing app tag. The testing app will still work fine on a 10.0 or 10.1 core system. So we do not need to artificially stop that from being possible.

Checklist:

• [x] Latest changes are published according to instructions in README.md