Unfortunately Kubernetes does not currently support resizing of PVCs that are managed by StatefulSets.

There are a few KEP and KEP PRs that will hopefully enable this in the future however: kubernetes/enhancements#661, kubernetes/enhancements#4650
And even when it is added, it will take a while for it to be available for users.

Luckily, starting in Kubernetes 1.24, PVCs themselves support expansion. Therefore all we need to do as the Solr Operator is to go and manually update the PVC resources, and then do a rolling restart of the Pods to ensure that they have an up-to-date view of the storage volume.