Drop support for PHP 7

[sneakyvv]

Problem

As #321 showed, there's an issue to upgrade the psr/simple-cache package since it changed its PHP constraint to >=8.0.0, and therefore this package's implementation of the CacheInterface in FileCache is no longer compatible with psr/simple-cache because it's using the new mixed type.

Solution

Drop support for PHP7. It's EOL since end of 11/2022, so now is the time to let it go.

NOTE

• This is of course a BC break, so that's why I'm targeting 6.x, but it seems to be behind on 5.x.
• Most changes in this PR have been made by running Rector (and verifying them)