In 2.x, the S3 repository accepted a `/` (forward slash) to start
the repositories.s3.base_path, and it used a different string splitting
method that removed the forward slash from the base path, so there
were no issues.
In 5.x, we removed this custom string splitting method in favor of
the JDK's string splitting method, which preserved the leading `/`.
The AWS SDK does not like the leading `/` in the key path after the
bucket name, and so it could not find any objects in the S3 repository.
This commit fixes the issue by removing the leading `/` if it exists
and adding a deprecation notice that leading `/` will not be supported
in the future in S3 repository's base_path.
bbf6e6d0bd