Update dependencies and some codestyle

[Jeroeny]

I'd also recommend:

• gitignore composer.lock, as committing it has no effect for libraries
• setting require: {"php": "^7.1 || ^8.0" to ^8.0 as 7.4 is EOL soon

[aboks]

My apologies for the delay. Unfortunately I cannot merge this as-is, as there are too many concerns mixed into one PR. As for the separate changes:

• As explained in an ealier PR, I'd like to keep an upper bound on the supported slevomat/coding-standard versions, based on actually released versions. If they ever break BC for their helpers in a minor version (which they have given themselves liberty to), we cannot retroactively restrict it. As an aside, my plan is to switch to https://github.com/PHPCSStandards/PHPCSUtils for this helper functionality once it hits stable. That would put an end to the Slevomat compatibility problems.
• I don't care that much for the code style changes, but I would have no objections in merging them.
• To remove the composer.lock we would need to make some changes to our GitHub Actions workflow, but that seems like a sensible idea.
• I'd rather not remove support for versions (either of PHP or slevomat/coding-standard) if it does not serve a purpose. If someone would like to continue using this library with PHP 7.3 that's fine by me if it works. If one such a version ever hinders new developments I'm fine with restricting the version range, but in my view it's not necessary to do this proactively.