Transition VMR sync pipeline to MI over AzDo pats

[dkurepa]

AzDo is removing PATs soon. We need to transition the only remaining pipeline we own to use a Managed Identity for authentication over a PAT.
This should be all of them https://dev.azure.com/dnceng/internal/_build?pipelineNameFilter=synch

[dkurepa]

We have created a new MI in the PCS prod resource group, VmrSyncPipeline.
I have added it to the dnceng org, and opened a workitem to add it to the DevDiv org https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2820146