Currently the Solr Operator is built with Kubebuilder 2.

Kubebuilder 3 was released a few months ago, and it's a good time to make the change after the v0.4.0 release.

This will help address the following issues:

• Expose operator metrics #307 - Metrics are supported by default starting in Kubebuilder 3
• Use v1 of the k8s networking and extension APIs #277 - Kubebuilder 3 upgrades the base controller-runtime version to one that supports the new networking APIs.

The refactor will have a much smaller footprint than the Kubebuilder 1 -> Kubebuilder 2 migration, but there are significant things to do.
Work items and issues will be tracked in the PR.