Setting importsToKeep to 1 in dataImportCronTemplates

[gaohoward]

What happened:
The dataImportCronTemplates currently do not specify the importsToKeep value which
defaults to 3. This is not ideal for users because it occupies more spaces (extra PVCs
up to 3) in default cases. Changing it to 1 makes more sense.

Ref https://issues.redhat.com/browse/CNV-69038

An alternative is to change the default to 1 (see this PR), however the downside is that it will break the API backward compatibility.

What you expected to happen:
Users who use the dataImportCronTemplate will get a initial importsToKeep value of 1.

How to reproduce it (as minimally and precisely as possible):
Use the template to deploy a dataImportCron CR.

Additional context:

Environment:

• KubeVirt version (use virtctl version): N/A
• Kubernetes version (use kubectl version): N/A
• VM or VMI specifications: N/A
• Cloud provider or hardware configuration: N/A
• OS (e.g. from /etc/os-release): N/A
• Kernel (e.g. uname -a): N/A
• Install tools: N/A
• Others: N/A

[orenc1]

It is possible to add importsToKeep: 1 in the DICTs in this file:
https://github.com/kubevirt/hyperconverged-cluster-operator/blob/main/assets/dataImportCronTemplates/dataImportCronTemplates.yaml
But I assume there was a reason of setting it to 3 by default in the first place.
@nunnatsa , WDYT ?