ci: verify extension against lowest dependencies

[Nikolas Evers (vintagesucks)]

follow-up to shopware/SwagMigrationAssistant#218

• the new Extension Verifier (lowest) job uses shopware-cli via shopware/github-actions/extension-verifier to validate the plugin against its lowest resolvable dependencies to catch future compat issues in CI
• the pre-existing #[Package('fundamentals@after-sales')] findings ignored in .shopware-extension.yml, 6.6.0 does not know that package label yet
• dependabot added for github-actions

example output from the first run of the job:

src/SwagPlatformDemoData.php
  17  error    [phpstan/argument.type]  Parameter #1 $package of attribute class Shopware\Core\Framework\Log\Package constructor expects 'administration'|'business-ops'|'buyers-experience'|'checkout'|'content'|'core'|'customer-order'|'data-services'|'innovation'|'inventory'|'merchant-services'|'sales-channel'|'services-settings'|'storefront'|'stranger-codes'|'system-settings', 'fundamentals@after…' given.

✖ 14 problems (14 errors, 0 warnings)