✨ Add :rocket: as a valid prefix for release PRs

[mjlshen]

Fixes #54 by adding :rocket: as a valid emoji prefix

Along the way I adjusted:

panic(fmt.Sprintf("unrecognized PR type %v", t))

to

panic("unrecognized PR type")

because I was getting this (rightful) compiler error when running tests:

fmt.Sprintf format %v with arg t causes recursive (sigs.k8s.io/kubebuilder-release-tools/notes/common.PRType).String method call

[mjlshen]

cc @g-gaston @furkatgofurov7 @vincepri as discussed today

[mjlshen]

/assign @vincepri

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mjlshen, vincepri

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[mjlshen]

@vincepri What do you think about a new release with this change so that cluster-api can pin to the new released version?

[vincepri]

done