Fix the cluster version to 1.28

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: houshengbo

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[codecov[bot]]

Codecov Report

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

Project coverage is 79.37%. Comparing base (8c9c8d0) to head (7ccaee4). Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1712 +/- ## ======================================= Coverage 79.37% 79.37% ======================================= Files 41 41 Lines 1857 1857 ======================================= Hits 1474 1474 Misses 279 279 Partials 104 104 ```

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[houshengbo]

/test eventing-upgrade-tests

[matzew]

/test eventing-upgrade-tests

[houshengbo]

/test eventing-upgrade-tests