[release pipeline] Target Preview 6 for 10.0 test pipeline

[mthalman]

The changes in https://dev.azure.com/dnceng/internal/_git/c02a7e43-a426-455b-bba9-335409c459ba/pullrequest/51142 (internal link) have a dependency on a newer job name that isn't available in the staging pipeline run that is referenced by the 10.0 test pipeline. This causes a test run failure due the artifact missing from the pipeline run. The staging pipeline run that is referenced needs to be updated to a Preview 6 run once that becomes available. That will have the required job name.

[NikolaMilosavljevic]

The root cause of this issue is tracked in #5291