PHP test matrix missing minimum WP version

[aaemnnosttv]

Feature Description

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Acceptance criteria

• The test matrix for PHP tests should include a job for testing against the minimum supported versions of PHP + WP

Implementation Brief

• Merge Update PHP test matrix #9785

Test Coverage

• No changes although this issue does increase coverage of tests running against the minimum supported version of WP

QA Brief

• No QA needed, changes can be verified during CR

Changelog entry

• N/A

[aaemnnosttv]

This is harder than it looks because of problems with PHP 7.4 and WP 5.2. I'm not sure we can actually do this.