Replace MinIO with Silo for object storage tests

What does this MR do and why?

Replace MinIO with Silo for object storage tests

The public minio/minio Docker images have been removed, which breaks the object_storage omnibus configuration. Switch to pgsty/silo, a community-maintained MinIO fork that keeps the same MINIO_ROOT_* environment variables, S3 API port and data directory, so the GitLab object store connection config is unchanged.

Fixes QA failing like https://gitlab.com/gitlab-org/gitlab/-/jobs/16962411586#L202

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How to set up and validate locally

CI should cover this well.

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