update PR icon template to match CAPI

We use the CAPI release-note tool and they switched the icon for other from
`:running:` to `:seedling:`.

This caused issues with the latest release
because the title for many PRs could not be automatically parsed and we
needed to fix the generated release notes manually.

The new text is a direct copy of CAPI PULL_REQUEST_TEMPLATE.md.

Author: apricote

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,5 @@
 <!-- please add a icon to the title of this PR (see https://sigs.k8s.io/cluster-api/VERSIONING.md), and delete this line and similar ones -->
-<!-- the icon will be either ⚠️ (:warning:, major or breaking changes), ✨ (:sparkles:, minor or feature additions), 🐛 (:bug:, patch and bugfixes), 📖 (:book:, documentation or proposals), or 🏃 (:running:, other) -->
+<!-- the icon will be either ⚠️ (:warning:, major or breaking changes), ✨ (:sparkles:, feature additions), 🐛 (:bug:, patch and bugfixes), 📖 (:book:, documentation or proposals), or 🌱 (:seedling:, minor or other) -->
 
 **What this PR does / why we need it**:
 

CONTRIBUTING.md

@@ -11,4 +11,4 @@ Kubernetes projects require that you sign a Contributor License Agreement (CLA)
 1. If your proposed change is accepted, and you haven't already done so, sign a Contributor License Agreement (see details above).
 1. Fork the desired repo, develop and test your code changes.
 1. Submit a pull request.
-1. All code PR must be labeled with ⚠️ (:warning:, major or breaking changes), ✨ (:sparkles:, minor or feature additions), 🐛 (:bug:, patch and bugfixes), 📖 (:book:, documentation or proposals), or 🏃 (:running:, other)
+1. All code PR must be labeled with ⚠️ (:warning:, major or breaking changes), ✨ (:sparkles:, feature additions), 🐛 (:bug:, patch and bugfixes), 📖 (:book:, documentation or proposals), or 🌱 (:seedling:, minor or other)