Support dot release from old release branches

[chaodaiG]

Current dot-release jobs only detects changes from latest release branch and creates new tags on top of latest release branches, there is no way to automate dot release from old release branches. This should be supported

This is part of #1782

[chaodaiG]

The easiest way to support this would be define RELEASE_BRANCH environment variable in Prow dot-release jobs, and creating a dot-release job for each old release branch will be needed.

Currently we support n-3 ... n release branches, i.e. at the time of creating this bug the latest release branch in serving is 0.13, so we'll have dot-release jobs for 0.10, 0.11, 0.12, and 0.13

[chaodaiG]

/cc @chizhg @grantr

[chizhg]

Fixed by #1828

/close

[knative-prow-robot]

@chizhg: Closing this issue.

In response to this:

Fixed by #1828

/close

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