Bump version to v2.28.0

[ydah]

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [-] Added tests.
• [-] Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).