Update to sacrebleu 2.3.1

[fhieber]

update default floor smoothing offset to 0.1 (which is the default in sacrebleu), removed a few unnecessary tests and adapted others.

Pull Request Checklist

• [x] Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]' until you can check this box.
• [x] Unit tests pass (pytest)
• [x] Were system tests modified? If so did you run these at least 5 times to account for the variation across runs?
• [x] System tests pass (pytest test/system)
• [x] Passed code style checking (./style-check.sh)
• [x] You have considered writing a test
• [x] Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• [x] Updated CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.