Fix PHP 8.4 deprecation warnings in 6.x

[W0rma]

From #726 (reply in thread):

I think we could do the same there, tag 6.0 then work on 6.1 bumping PHP and doing all the changes

This PR uses a similar strategy as in #731 but for 6.x:

• bump minimum PHP version to 7.1
• fix PHP deprecation warnings

[DannyvdSluijs]

Hi @W0rma

Thanks for the contribution. I was curious why you're targeting the 6.0.x branch? As that branch is for patch release for version6.x and we should not drop support of dependency versions in a patch release.

The changes (besides the one in .php-cs-fixer.dist.php) are already part of the master branch. So all it needs is work towards a release of a new mayor version. The discussion you linked has a todo list on what (ideally) needs to be done.

Also if you think the changes in .php-cs-fixer.dist.php are needed can you create a PR adding those changes to the master branch?

[W0rma]

So all it needs is work towards a release of a new mayor version.

@DannyvdSluijs Will the next release be a major (7.0) or a minor (6.1) version?

I wasn't sure if master already contains BC breaks and my aim was to fix the deprecation warnings for the next 6.x minor version.