Migrate away from LocalStack

### Description / Background

LocalStack no longer allows download of free version without authentication. It looks like it's still working for us because we pin an older version of the Docker image. If we wanted to upgrade it doesn't seem like we'd be able to right now.

See the notice here:
https://hub.docker.com/r/localstack/localstack/

We'd like to decide whether we can continue to use LocalStack or migrate to an alternative.

### Technical Notes / Implementation Details

Here are some options:

https://github.com/getmoto/moto - This seems relatively mature, and has a Docker version we could use in Java.
https://github.com/floci-io/floci - This is very new, but looks like it can do what we need

Review the repository's current LocalStack Docker image usage and the Java integration points that depend on it. Compare the proposed Moto and Floci alternatives against the existing needs, then confirm that the selected replacement works without the blocked LocalStack download path.

Written by the indexing model from the issue text.