Test MongoDB appender against Docker image

Right now (after #1589), we are using Embedded MongoDB to:

- download a MongoDB distribution on our system,
- run it with the correct parameters,
- test the MongoDB appender.

This procedure has some problems:

1. we cannot run tests in parallel on a clean system (each test tries to download MongoDB at the same time),
2. the MongoDB distribution depends on some system libraries that are not always present.

Therefore we should move the tests to use [TestContainers](https://testcontainers.com/) or an equivalent solution and the official [Docker `mongo`](https://hub.docker.com/_/mongo/) images.

Locate the existing tests that use Embedded MongoDB and review how the MongoDB distribution is downloaded, started, and configured. Replace that test setup with TestContainers or an equivalent approach using the official mongo Docker images, then verify the MongoDB appender tests can run in parallel on a clean system.

Written by the indexing model from the issue text.