Cannot delete ingress.ui's annotations once created

**What version of operator are you using?**
commit 561cf47d783c368fd8795acb82a5a39099a35984 (HEAD -> master)

**What operating system and processor architecture are you using (`kubectl version`)?**
Ubuntu. 20.04

kubectl version Output

$ kubectl version

Client Version: version.Info{Major:"1", Minor:"24", GitVersion:"v1.24.0", GitCommit:"4ce5a8954017644c5420bae81d72b09b735c21f0", GitTreeState:"clean", BuildDate:"2022-05-03T13:46:05Z", GoVersion:"go1.18.1", Compiler:"gc", Platform:"darwin/amd64"}

Kustomize Version: v4.5.4

Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.9", GitCommit:"6df4433e288edc9c40c2e344eb336f63fad45cd2", GitTreeState:"clean", BuildDate:"2022-05-19T19:53:08Z", GoVersion:"go1.16.15", Compiler:"gc", Platform:"linux/amd64"}

WARNING: version difference between client (1.24) and server (1.22) exceeds the supported minor version skew of +/-1

**What did you do?**

We are trying to delete the annotations of the `ingress.ui` by setting the `ingress.ui.annotations` to `null`
I first deployed the operator using the example yaml file and crd yaml file.
Then I created the cluster by applying the following custom resource file:

```
apiVersion: crdb.cockroachlabs.com/v1alpha1
kind: CrdbCluster
metadata:
name: test-cluster
spec:
additionalLabels:
crdb: is-cool
dataStore:
pvc:
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
volumeMode: Filesystem
image:
name: cockroachdb/cockroach:v21.2.10
ingress:
ui:
annotations:
mykey: myvalue
host: localhost
nodes: 3
resources:
limits:
cpu: 2
memory: 2Gi
requests:
cpu: 100m
memory: 1Gi
tlsEnabled: true
```

(All files were applied using `kubectl apply -f -n cockroach-operator-system`)

Then I deleted the `ingress.ui.annotation` field (basically, deleting "annotations"), as follows:

```
apiVersion: crdb.cockroachlabs.com/v1alpha1
kind: CrdbCluster
metadata:
name: test-cluster
spec:
additionalLabels:
crdb: is-cool
dataStore:
pvc:
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
volumeMode: Filesystem
image:
name: cockroachdb/cockroach:v21.2.10
ingress:
ui:
annotations: null
host: localhost
nodes: 3
resources:
limits:
cpu: 2
memory: 2Gi
requests:
cpu: 100m
memory: 1Gi
tlsEnabled: true
```

**What did you expect to see and what did you see instead?**

I expect to see annotations being deleted, but they are still present.

**Root cause**
The way annotations are updated is merging new annotations into old ones, as [this function](https://github.com/cockroachdb/cockroach-operator/blob/dda0a6308b0f1c64d41e0851a5ef8cb60cd98a37/pkg/kube/helpers.go#L340) suggests. However, when a field is deleted (i.e. being changed to null), the key and value pair will not show up in the new annotations, resulting in the old annotations leaving unchanged.