Remove PHP version 5.6 from CI

[gabrielcld2]

The Cloudinary plugin isn't officially supporting PHP 5.6 anymore, so removing the PHP 5.6 CI testing.

Approach

• Removing the 5.6 version from the GH workflow CI
• Only 7.4 and 8.3 remains in the CI PHP versions matrix

QA notes

• On GitHub, go to the actions run for this change at https://github.com/cloudinary/cloudinary_wordpress/actions/runs/21856104008
• Check the jobs have run successfully, and they only contain the builds for PHP versions 7.4 and 8.3.