docs: document test framework variable function renaming

Author: chrischdi

docs/book/src/developer/providers/migrations/v1.9-to-v1.10.md

@@ -17,4 +17,9 @@ maintainers of providers and consumers of our Go API.
 
 ### Other
 
+- The following functions for `E2EConfig` in `sigs.k8s.io/cluster-api/test/framework/clusterctl` got renamed ([#11743](https://github.com/kubernetes-sigs/cluster-api/pull/11743)):
+  - `E2EConfig.GetVariable` is now `E2EConfig.MustGetVariable`
+  - `E2EConfig.GetInt64PtrVariable` is now `E2EConfig.MustGetInt64PtrVariable`
+  - `E2EConfig.GetInt32PtrVariable` is now `E2EConfig.MustGetInt32PtrVariable`
+
 ### Suggested changes for providers