Drop support for PHP 8.0 | Add support for PHP 8.4

[sprain]

• Drops support for PHP 8.0
• Adds support for PHP 8.4
• Adds modernizations available from PHP 8.1
• Adds policy on how long PHP versions and dependencies are supported

[kohlerdominik]

This library supports a PHP version as long as it accounts for at least 10% of current installations

@sprain While this stat is interessting for a general overview, I wouldn't assume its use based on the install count (read: composer install). There might be pipelines that run on every commit and therefore dozens of installs per week per developer (like ours). This does not really represent it's real "use".

edit: also, if we're developing small features, there are a lot more commits (=installs) than if we develop on a huge feature.

[sprain]

@kohlerdominik Thanks for the feedback.

I would handle this number flexibly. If based on installs one can assume that the number of real use cases is below 10%, dropping a version becomes an option. It serves an indicator for decision making.

In the past I dropped a few dependencies too soon, which caused issues for some users. I accept the challenge to avoid repeating this :)