ci: standardise test matrix and WP_ENV_CORE format

[GaryJones]

Summary

• Converts the full matrix strategy to the standard min/max include pattern for consistency with other Automattic plugins
• Reduces CI runs from 4 to 2 whilst still testing the important version combinations (WP 6.4 + PHP 7.4, WP master + PHP latest)
• Adds WP_ENV_CORE as a job-level env variable
• Removes inline WP_ENV_PHP_VERSION from the wp-env start command

Test plan

• [ ] CI passes on this PR

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