Verify all supported Drupal branches and PHP combinations in CI

Problem/Motivation

The alpha2 CI matrix pins Drupal 10.3.* and resolves ^11. It does not explicitly exercise currently supported Drupal 10.6.x and 11.3.x alongside 11.4.x. Passing an old compatibility fixture is not evidence for every supported branch.

Proposed resolution

Extend the matrix using the current upstream lifecycle. Retain useful compatibility-floor coverage; do not narrow advertised support to avoid test work. Keep receiver-only and supported Mailer Plus paths represented.

Acceptance criteria

• Run kernel, HTTP, upgrade, reconciliation and Drush checks on Drupal 10.6.x, 11.3.x and 11.4.x, revisiting this list at implementation time.
• Exercise the module's declared PHP floor and a current compatible PHP version with explicit constraints and record resolved versions.
• Preserve Mailer Plus 1.6 and 2.x coverage where Composer compatibility permits, and document any real upstream constraints.
• Keep current supported jobs subject to normal dependency-security checks; any legacy-fixture exception must remain isolated.
• Update docs/integration-verification.md with reproducible results.

Scope and dependencies

Stable-release acceptance task under #3621224: Complete the 1.0.0 stable release verification gates. Builds on #3621137: Expand integration coverage to routing, mail transport, forms and Drush; the completed original issue remains fixed.

References

• https://www.drupal.org/about/core/policies/core-release-cycles/schedule?showAll=true