ci: test against WordPress 6.7

[justlevine]

What

This PR bumps the Tested up to header and various CI variables to WordPress 6.7.

[changeset-bot]

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Latest commit: 189276d

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[justlevine]

@theodesp based on the failed tests, WP6.7+PHP7.4 need to be added to the workflow matrix's exclude list (~L28)

I'm afk until this evening your time, but commits on the branch should be open if you want to patch + merge

[justlevine]

@theodesp 54c0746 should the trick