rfc: remove kubebuilder-style emoji PR classification

[embik]

I would like to make a change to our PR process and I wanted to document the rationale for that to solicit some input.

Context

Pull requests in this project need to be categorized in some way so that a meaningful changelog can be generated. Right now, we have a pull request template that asks for two things:

1. Add an emoji to the PR title (this is the "kubebuilder" way of doing things and is validated by a GitHub action.
2. A release-notes block in the PR description to describe the user-facing change of the PR (this is the Kubernetes way of doing release notes).

We use the Kubernetes release tooling release-notes to generate release changelogs.

Problem

Simply said: release-notes does not understand emoji titles. We essentially ended up with a weird mix of kubebuilder and kubernetes/kubernetes release tooling, which requires the release manager to go over all PRs and add a kind/ label to merged PRs to get classification in the changelog generated by release-notes.

I've looked at the release tooling for kubebuilder and it seems much simpler than release-notes, which means that we cannot have a PR title and a user-facing release note (while PR titles might make sense mostly to other kcp contributors, a changelog generally has to appeal to a different audience, the kcp user).

Proposal

I'm in favour of moving fully to the Kubernetes release-notes process (since k/k is also our direct upstream), which means we will remove the emoji-style PR titles and instead ask contributors to add a /kind command to their PR, which will categorise the PR as bugfix, feature, or some other things (e.g. a chore).

[embik]

This has been implemented! Please add a /kind <type of PR> line to your PRs from now on. The new PR template has a selection of them for your choice.