[release-1.13] Make the branch configurable for Serving

[knative-prow[bot]]

@houshengbo: The label(s) kind/<kind> cannot be applied, because the repository doesn't have them.

In response to [this](https://github.com/knative-extensions/serving-progressive-rollout/pull/180): > > > > ># Changes > > > >- >- >- > > >/kind > > >Fixes # > > > >**Release Note** > > >```release-note > >``` > >**Docs** > > >```docs > >``` > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: houshengbo

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/knative-extensions/serving-progressive-rollout/blob/release-1.13/OWNERS)~~ [houshengbo] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[codecov[bot]]

Codecov Report

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

Project coverage is 48.83%. Comparing base (ddf75b0) to head (cab2468). Report is 6 commits behind head on release-1.13.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.13 #180 +/- ## ================================================ - Coverage 49.42% 48.83% -0.60% ================================================ Files 23 25 +2 Lines 2280 1894 -386 ================================================ - Hits 1127 925 -202 + Misses 1109 924 -185 - Partials 44 45 +1 ```

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