Update Datumaro version to 1.6.0rc1

[vinnamkim]

Summary

• We still need datumaro==1.6.0rc2 to address:
  1. Change RoIImage and MosaicImage to have np.uint8 dtype as default datumaro#1245
  2. Enable image backend and color channel format to be selectable datumaro#1246
• But, let's merge this until publishing it.

How to test

I believe our existing tests can cover this.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added e2e tests for validation.
• I have added the description of my changes into CHANGELOG in my target branch (e.g., CHANGELOG in develop).​
• I have updated the documentation in my target branch accordingly (e.g., documentation in develop).
• I have linked related issues.

License

• I submit my code changes under the same Apache License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
# SPDX-License-Identifier: Apache-2.0

[sungmanc]

LGTM