Update version for the next release (v0.28.0)

[meili-bot]

This PR is auto-generated.

The automated script updates the version of meilisearch-ruby to a new version: "v0.28.0"

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (1875bac) to head (f11ecfe).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #543 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 10 10 Lines 677 677 ========================================= Hits 677 677 ```

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[meili-bors[bot]]

Build succeeded:

• integration-tests (ruby 3.1)
• integration-tests (ruby 3.2)
• integration-tests (ruby 3.3)
• linter-check