Declare supported PHP versions explicitly and run tests on all supported versions

[gsteel]

Description

Instead of declaring support for unreleased PHP versions using ^8.0, declare the versions that are supported in composer.json. This will prevent installation on PHP 8.6 which you currently cannot feasibly declare support for.

Also amends the test matrix to run on all PHP versions and operating systems.

Checklist

• [NA] I have understood, reviewed, and tested all AI outputs before use
• [NA] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated

[sgiehl]

Thanks for the contribution!