Repair wp/6.1 branch Actions: Remove the stale Composer PHP platform override - #84251

Part of #82932.

What?

Removes composer.json's config.platform.php value ("7.4") and replaces it with an explicit require.php constraint of ">=5.6.20", letting Composer auto-detect the PHP version it is actually running under.

Why?

config.platform.php tells Composer to resolve dependencies as though the host were that exact PHP version, no matter what PHP is really in use, and on this branch the hard-coded 7.4 is now stale:

• The unit-php legs install on the runner, with shivammathur/setup-php at the leg's PHP version (5.6, 7.0, 7.4, 8.0, 8.2) and then ramsey/composer-install (added by Repair wp/6.1 branch Actions: Add a PHP version test matrix #84248). The hard-coded 7.4 makes every leg resolve as though it ran PHP 7.4, so the 5.6 and 7.0 legs get dependencies they cannot run.
• The phpcs job from Repair wp/6.1 branch Actions: Run PHP coding standards on the runner instead of inside wp-env #84250 installs on the runner at PHP 7.4, where the value is redundant.
• Anywhere else, such as a contributor's local composer install on another PHP version, the hard-coded 7.4 silently resolves dependencies against the wrong PHP version.

So the static value is wrong for most of the CI matrix and a trap outside CI. Removing it and declaring the real floor with require.php is what composer.json should have said all along.

">=5.6.20" is the correct floor for this branch: it is the minimum PHP version WordPress required from 5.2 until PHP 5.6 support was dropped in 6.3 (WordPress/wordpress-develop@a66e27e), and PHP 5.6 is the lowest version the matrix tests. Verified by simulating dependency resolution for every matrix leg with composer update --dry-run against realistic patch versions (5.6.40, 7.0.33, 7.4.33, 8.0.30, 8.2.26), plus one version below the floor (5.5.38) to confirm the constraint is genuinely enforced.

How?

This is a port of the approach taken upstream in commit d2eb077 ("Unit testing: Allow Composer to auto-detect PHP version", WordPress/gutenberg#73358), already present on wp/6.9 and applied to wp/6.8 through wp/6.2. Each branch uses its own WordPress release's minimum PHP version as the floor; wp/6.2 (#84227) uses the same >=5.6.20.

Unlike wp/6.8–wp/6.2, this branch has no "Override PHP version in composer.json" step in unit-test.yml, so only composer.json changes.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

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