Add PHP 8.4/8.5 images and extend WordPress support through 7.0

[Copilot]

This updates the published image matrix to cover newer PHP and WordPress releases. It adds image definitions for PHP 8.4 and 8.5, and extends supported WordPress versions through 6.9 and 7.0.

• Build matrix

  • Extend .github/workflows/image.yml to publish images for:
    • WordPress 6.9, 7.0
    • PHP 8.4, 8.5
  • Move the latest tag forward to the newest WordPress line on the default PHP 7.4 image.

• Runtime images

  • Add Dockerfile.php8.4 based on Alpine with the PHP 8.4 package set.
  • Add Dockerfile.php8.5 for early PHP 8.5 support using Alpine packages that provide the 8.5 toolchain.
  • Keep the existing image shape and bootstrap flow unchanged: WordPress download, wp-phpunit download, MySQL setup, and PHPUnit entrypoint.

• Documentation

  • Refresh README version lists and tag examples to include PHP 8.4/8.5 and WordPress 6.9/7.0.
  • Update the sample matrix configuration to reflect currently supported version ranges.

Example tag usage:

docker run --rm -v "$PWD:/code" humanmade/plugin-tester:wp-7.0-php8.5

[kadamwhite]

@rmccue This has revealed that we also should probably bump our default from 7.4 to 8.2 (minimum); but since that's a breaking change and this is an incremental addition, I'd like to do that as a separate PR.