chore: Drop support for PHP 7.4

[DerDreschner]

This pull request drops support for PHP 7.4 in order to modernize the library.

[Copilot]

Pull request overview

Drops PHP 7.4 support across project configuration to align tooling, CI, and documentation with PHP 8+.

Changes:

• Update Composer PHP requirement to PHP ^8.0 and pin dev dependency resolution to PHP 8.0 via config.platform.
• Configure PHPStan to analyze against a PHP 8.0–8.5 target range.
• Remove PHP 7.4 from CI and update README minimum PHP version for master.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
phpstan.neon	Sets PHPStan target PHP version range starting at 8.0.
composer.json	Drops PHP 7.4 requirement and pins platform PHP to 8.0 for consistent dependency resolution.
README.md	Updates documented minimum PHP version for master to PHP 8.0.
.github/workflows/ci.yml	Removes the PHP 7.4 CI job.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.70%. Comparing base (a4fd239) to head (1c16720).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #134   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files           1        1           
  Lines         174      174           
=======================================
  Hits          170      170           
  Misses          4        4           

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[phil-davis]

Issue #137
For discussion, what minimum PHP 8.* do we support?

[phil-davis]

And we need to decide if we move further forward to minimum PHP 8.2 or?

[phil-davis]

We need to run code-style somewhere.
So change this job from 7.4 to 8.0
And remove 8.0 from the array at line 15.

[phil-davis]

I used this code, with needed changes, in #140 to demonstrate supporting just PHP 8.2 and up.

We need to decide what is the minimum PHP version that we want to support now.