Use low-privilege secrets variable group for DockerHub access (#1794)

Author: lbussell

eng/common/templates/variables/dotnet/secrets-unofficial.yml

@@ -1,5 +1,5 @@
 variables:
-- group: DotNet-Docker-Secrets-Unofficial
+- group: DotNet-Docker-Secrets-Low
 
 - name: dockerHubRegistryCreds
-  value: --registry-creds 'docker.io=$(dotnetDockerHubBot.userName);$(BotAccount-dotnet-dockerhub-bot-PAT)'
+  value: --registry-creds 'docker.io=$(dotnet-dockerhub-bot-username);$(dotnet-dockerhub-bot-pat-low)'