Repair wp/6.6 branch Actions: update test-php job's PHP version matrix

[desrosj]

Part of #82784.

What?

Updates unit-test.yml's test-php job PHP version matrix from 7.2, 7.3, 7.4, 8.0, 8.1, 8.2 to 7.2, 7.4, 8.0, 8.3.

Why?

Per WordPress core's PHP compatibility policy, WordPress 6.6 supports PHP 7.2 through 8.3 (7.0/7.1 dropped at this release). This project's convention is to test only the lowest and highest minor version within each supported PHP major version (7.x, 8.x), not every minor in between. The previous matrix was stale: it never picked up 8.3 support and tested several minors (7.3, 8.1, 8.2) this project doesn't individually cover under that convention.

Use of AI Tools

This PR was authored with the help of Claude Code.

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