external-plugins/cherrypicker: include PR description as comment

[AlexNPavel]

This PR prepends the PR description to the comments list, allowing users to specify branches that they wish to cherrypick to when they first create the PR.

This change fixes the cherrypick chain functionality introduced in #136.

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[AlexNPavel]

/cc @bradmwilliams

[k8s-ci-robot]

@AlexNPavel: GitHub didn't allow me to request PR reviews from the following users: bradmwilliams.

Note that only kubernetes-sigs members and repo collaborators can review this PR, and authors cannot review their own PRs.

In response to [this](https://github.com/kubernetes-sigs/prow/pull/174#issuecomment-2138381214): >/cc @bradmwilliams 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.

[k8s-ci-robot]

@bradmwilliams: changing LGTM is restricted to collaborators

In response to [this](https://github.com/kubernetes-sigs/prow/pull/174#pullrequestreview-2086926709): > 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.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AlexNPavel, bradmwilliams, matthyx

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[cmd/external-plugins/cherrypicker/OWNERS](https://github.com/kubernetes-sigs/prow/blob/main/cmd/external-plugins/cherrypicker/OWNERS)~~ [matthyx] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment