Pin Testbench per Laravel major in CI

[sandervanhooft]

Summary

• drop Laravel 11 from the supported Composer constraints and CI matrix
• keep Composer security advisory blocking enabled in CI
• keep Laravel 12 jobs on Testbench 10.x and Laravel 13 jobs on Testbench 11.x
• preserve the compact PHP x Laravel matrix instead of listing every supported combination manually
• update actions/checkout to v6 so workflow jobs run the checkout action on Node 24

Why

Laravel 11 is no longer receiving Laravel security fixes, and Composer 2.10 now blocks the Laravel 11 framework versions selected by the Testbench 9.x lane because Packagist reports active security advisories.

Rather than disabling Composer advisory blocking, this PR stops advertising and testing Laravel 11 support. The remaining Laravel 12 and 13 compatibility lanes resolve with security checks intact.

The checkout action update also removes the GitHub Actions annotation about Node.js 20 actions being deprecated.

Verification

• composer validate --strict
• Composer 2.10.1 secure resolver dry-run
• GitHub Actions test workflow is green for both triggered runs