fix php 8.5 deprecations

[diosmosis]

Description

As title. Also introduces a matrix for the phpunit github actions workflow (testing against php 7.4 and 8.5).

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

[textagroup]

All seems to work OK.
Ran an AI review which brought up the one minor issue

• Found one issue:

• P2 – stale headers can be reused after a failed request.
  In classes/WP_Piwik/Request/Rest.php:153, libs/matomo-php-tracker/MatomoTracker.php:2217, and proxy/proxy.php:426, PHP 8.4+ reads http_get_last_response_headers() without first clearing the
  previously stored response. If multiple stream requests occur in one PHP request and a later request fails before receiving headers, the previous response’s status/cookies may be treated as belonging
  to the failed request. PHP provides http_clear_last_response_headers() specifically for clearing this state. PHP documentation

  Add a clear immediately before each file_get_contents() call, and add a regression test covering successful request followed by transport failure.