Simplify image CI and sunset PHP 8.1

[dbpolito]

• Retire all PHP 8.1 variants from templates, builds, and publishing. Existing Docker Hub tags remain available without further updates; 28 PHP 8.2–8.5 tags remain.
• Consolidate image smoke tests and Alpine/Debian matrices. Fork PRs build and test without secrets or publishing; master releases retain base-before-derived amd64/arm64 publishing.
• Update GitHub Actions, group Dependabot updates weekly, and verify generated images match their templates. Includes the upgrades proposed in Bump peter-evans/repository-dispatch from 3 to 4 #123, Bump actions/checkout from 4 to 6 #124, Bump docker/login-action from 3 to 4 #125, Bump docker/setup-qemu-action from 3 to 4 #126, and Bump docker/setup-buildx-action from 3 to 4 #127.
• Fix Docker Hub README sync inputs and refresh tag documentation and examples.

Downstream builds now warn and skip when DISPATCH_TOKEN is missing. Restore that secret to enable dispatches after publishing.

The full revised build matrix still needs to run in this PR; local smoke testing covered the existing 8.4-nginx image.