No code changed and nothing on the wire moved. composer.json now requires
php: >=8.1, which is the whole of it.

This exists to let the connector support Craft 4. Craft 4 runs on PHP
8.0.2+, and the sites still on Craft 4 are disproportionately the ones
still on an older PHP.

8.1 rather than Craft 4's own 8.0.2 floor: readonly appears on thirteen
promoted properties across CanonicalRequest, CanonicalResponse and
SchemaValidator, and those are the objects a signature is computed over.

The test matrix does not cover 8.1 and cannot -- Pest 3 requires ^8.2, so
composer install will not resolve there. phpstan.neon sets phpVersion
{min: 80100, max: 80400} instead, so the source is analysed across the
whole supported range.

Also carries the removal of .github/allowed_signers, and a fix making the
artifact tamper test tamper on every run rather than 255 in 256.