Update gp2 to gp3 in AWS storage class example (#674)

It's been some time since gp3 has been available, with better
performance and pricing than gp2. We should update our example to reduce
the chances of a customer deploying with gp2.


https://docs.aws.amazon.com/eks/latest/userguide/create-storage-class.html

### Checklist

- [ ] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

Tested in test instance

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Author: marcleblanc2

charts/sourcegraph/examples/aws/override.yaml

@@ -10,7 +10,7 @@ frontend:
 
 storageClass:
   create: true
-  type: gp2 # This configures SSDs (recommended).
+  type: gp3 # This configures SSDs (recommended).
   provisioner: ebs.csi.aws.com
   volumeBindingMode: WaitForFirstConsumer
   reclaimPolicy: Retain