Add migration notes

Signed-off-by: Stefan Büringer [email protected]

Author: sbueringer

docs/book/src/developer/providers/migrations/v1.4-to-v1.5.md

@@ -3,7 +3,7 @@
 This document provides an overview over relevant changes between Cluster API v1.4 and v1.5 for
 maintainers of providers and consumers of our Go API.
 
-## Minimum Go version
+## Go version
 
 - The Go version used by Cluster API is Go 1.20.x
 
@@ -12,6 +12,8 @@ maintainers of providers and consumers of our Go API.
 **Note**: Only the most relevant dependencies are listed, `k8s.io/` and `ginkgo`/`gomega` dependencies in Cluster API are kept in sync with the versions used by `sigs.k8s.io/controller-runtime`.
 
 - sigs.k8s.io/kind: v0.17.x => v0.19.x
+- sigs.k8s.io/controller-runtime: v0.14.x => v0.15.x
+- sigs.k8s.io/controller-tools: v0.11.x => v0.12.x
 
 ## Changes by Kind
 
@@ -36,3 +38,21 @@ maintainers of providers and consumers of our Go API.
 ### Suggested changes for providers
 
 - 
+
+## Notes about the controller-runtime bump
+
+This section shares our learnings of bumping controller-runtime to v0.15 in core Cluster API. It highlights the most relevant changes and pitfalls
+for Cluster API providers. For the full list of changes please see the [controller-runtime release notes](https://github.com/kubernetes-sigs/controller-runtime/releases/tag/v0.15.0).
+
+* Webhooks can now also return warnings, this requires adding an additional `admission.Warnings` return parameter to all webhooks. 
+* Manager options have been refactored and old fields have been deprecated.
+* Manager now has a builtin profiler server which can be enabled via `Options.PprofBindAddress`, this allows us to remove our profiler server.
+* Controller builder has been refactored, this requires small changes to our controller setup code.
+* The EventHandler interface has been modified to also take a context, which affects our mapping functions (e.g. `ClusterToInfrastructureMapFunc`).
+* Controller-runtime now uses a lazy restmapper per default, i.e. API groups and resources are only fetched when they are actually used.
+  This should drastically reduce the amount of API calls in clusters with a lot of CRDs.
+* Some wait utils in `k8s.io/apimachinery/pkg/util/wait` have been deprecated. The migration is relatively straightforward except that passing in `0` 
+  as a timeout in `wait.PollUntilContextTimeout` is treated as a timeout with 0 seconds, in `wait.PollImmediateWithContext` it is interpreted as infinity.
+* The fake client has been improved to handle status properly. In tests that write the CRD status, the CRDs should be added to the fake client via `WithStatusSubresource`.
+
+For reference, please see the [Bump to CR v0.15 PR](https://github.com/kubernetes-sigs/cluster-api/pull/8007) in core Cluster API.