Update release process doc to use the cherrypicker plugin for patch releases

[AlanGreene]

Changes

The cherrypicker plugin was enabled for the Dashboard repo in tektoncd/plumbing#1801

/kind misc

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Docs included if any changes are user facing
• Tests included if any functionality added or changed
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Release Notes

NONE

[briangleeson]

/lgtm

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