ci: verify extension against lowest dependencies

[Nikolas Evers (vintagesucks)]

follow-up to shopware/SwagMigrationMagento#62

• AttributeReader::cleanupColumns() used getReferencingColumnNames() and UnqualifiedName (DBAL 4.3+) since fix: phpstan errors #67, so attribute reads through the Shopware local gateway failed on installs whose lockfiles contain an older DBAL
• 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 issues like this in CI in the future
• shopware/storefront added to suggest, the verifier installs suggested packages and needs them to resolve the theme classes RunService pulls in
• the two Symfony DI configurator findings ignored in .shopware-extension.yml instead of fixed in code, the only in-code fix would be moving src/DependencyInjection/*.php to src/Resources/config/ which felt too heavy here

example output from the first run of the job:

phpstan.neon
  0  error    [phpstan/error]  Internal error: Class Doctrine\DBAL\Schema\Name\UnqualifiedName was not found while trying to analyse it - discovering symbols is probably not configured properly. while analysing file /tmp/analyse-extension-1320630473/src/Profile/Shopware/Gateway/Local/Reader/AttributeReader.php

✖ 10 problems (5 errors, 5 warnings)