Remove constraints in SylvaUnitsReleaseTemplate.spec.sylvaUnitsSource

In !430 (merged) we made SylvaUnitsReleaseTemplate fields optional, but the sylvaUnitsSource fields are not:

❯ cat test-surt.yaml 
apiVersion: unitsoperator.sylva/v1alpha1
kind: SylvaUnitsReleaseTemplate
metadata:
  name: test
  namespace: swct-team-a
spec:
  clusterType: workload
  sylvaUnitsSource:
    type: git
    url: https://gitlab.com/sylva-projects/sylva-core.git
  valuesFrom:
    - name: managed-workload-clusters-settings
      type: ConfigMap
      valuesKey: values

on ⛵ management-cluster  ~ 
❯ k apply -f test-surt.yaml 
The SylvaUnitsReleaseTemplate "test" is invalid: spec.sylvaUnitsSource: Invalid value: "object": for type git, one of tag/branch/commit is required

This is causing troubles in sylva-projects/sylva-core!5883 (merged) as described in sylva-projects/sylva-elements/ci-tooling/ci-deployment-values!295 (comment 2918432125)