Tests: Update to supported WP version 6.8

[masteradhoc]

Fixes #767

What?

Update the GitHub Actions test matrix to run CI against WordPress 6.8 instead of 6.7.

Why?

WordPress 6.8 is the minimum supported WP release of this repo. Testing against the trunk, latest stable version and our minimum supported WP version ensures continued compatibility and keeps CI coverage aligned with supported WordPress releases.

How?

Replace the 6.7 entry in the GitHub Actions matrix with 6.8.
The workflow already resolves WordPress versions via official wordpress.org release ZIPs, so no additional changes or dependencies are required.

Testing Instructions

1. Open the GitHub Actions run for this PR.
2. Verify that jobs run with:
3. WP 6.8 (instead of 6.7) across supported PHP versions.
4. Check the workflow logs for the WordPress version output to confirm 6.8.x is installed.

Screenshots or screencast

Changelog Entry

Changed – CI tests now run against WordPress 6.8 instead of 6.7.

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[jeffpaul]

@masteradhoc please ensure you also update the following to 6.8 then:

two-factor/two-factor.php

Line 14 in 8b998d4

* Requires at least: 6.7