[ECP-8518] Unset amqp in config to use default queue connection

[factorin-j]

Description

• remove amqp in queue xml config to use Magento's default (db) connection if amqp service is not installed.
• include additional admin message when using 'queue' notification processing
• add unit tests

Tested scenarios

• Tested using 2 Magento setups:
  • bin/magento setup:install with opensearch/elasticsearch only
  • bin/magento setup:install with --amqp-* options included (RabbitMQ installed).
• Unit test output:

docker compose exec -it php vendor/bin/phpunit -c dev/tests/unit/phpunit.xml app/code/Adyen/Payment/Test/Unit/Model/Queue/
PHPUnit 9.5.24 #StandWithUkraine

...                                                                                                                                                                                                                                                3 / 3 (100%)

Time: 00:00.047, Memory: 22.00 MB

OK (3 tests, 8 assertions)

Fixes #2031