ci(build-system-tests): replace Angular 19 mega-app with Angular 20

[pranavosu]

Description of changes

The publish-next workflow has been failing on every push to main since the Angular 20 upgrade (#7047) landed. The publish job itself succeeds, but the post-publish canary job build-test / build (angular, 19, angular-cli, 19, npm, 24) fails during npm install:

npm error code ERESOLVE
npm error While resolving: angular-19-angular-cli-19-ui-next@0.0.0
npm error Found: @angular/common@19.2.25
npm error   @angular/common@"^19.2.0" from the root project
npm error Could not resolve dependency:
npm error peer @angular/common@">= 20.0.0" from @aws-amplify/ui-angular@0.0.0-next-...

#7047 raised the @aws-amplify/ui-angular peer range to >= 20.0.0, but the build-system-test matrix still scaffolds an Angular 19 mega-app, so the published @next package can no longer be installed there. install-with-retries burns all 4 attempts and the job fails.

This replaces the Angular 19 matrix entry with Angular 20. Angular 19 reached end of life on 2026-05-19, so there is no reason to keep it in the canary matrix. Pinning to 20 keeps coverage of the floor of the declared peer range, alongside the existing latest (v22) entry.

No script changes are needed: both mega-app-copy-files.sh and mega-app-install.sh branch on >= 21 for the newer bootstrap config and the explicit zone.js dependency, so an Angular 20 app takes the same path Angular 19 did.

Issue #, if available

N/A

Description of how you validated changes

• Verified the failing job and root cause in run 29840776966 and in the two prior publish-next failures, all failing on the same Angular 19 job. The last green publish-next run predates chore(angular): upgrade @aws-amplify/ui-angular to Angular 20 #7047.
• Confirmed .github/workflows/reusable-build-system-test.yml is the only place the Angular matrix is defined (build-system-test.yml delegates to it, and the react-native reusable workflow has no Angular entries).
• Parsed the workflow YAML to confirm the matrix resolves to Angular latest + 20 with no remaining 19 entries.
• prettier --check passes on the modified file.
• The canary build for the new angular-20-angular-cli-20 mega-app runs on merge to main.

Checklist

• Have read the Pull Request Guidelines
• PR description included
• PR title and commit messages follow conventional commit syntax
• If this change should result in a version bump, changeset added — not applicable, CI-only change with no package code affected

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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