Fix velero tester image references

https://repo1.dso.mil/big-bang/apps/cluster-utilities/velero/-/blob/2e431e64ea2a190e7edf7ed7b6713d3774fa6ddc/chart/Chart.yaml#L40-42

https://repo1.dso.mil/big-bang/apps/cluster-utilities/velero/-/blob/2e431e64ea2a190e7edf7ed7b6713d3774fa6ddc/renovate.json#L57-65

Currently these two spots are pointed to an outdated version of the velero tester image. The new image is hosted at registry1.dso.mil/bigbang-ci/velero-tester and we should reference this in those two spots.