Add .NET Monitor 8 to version update pipeline (#4243)

Author: jander-msft

eng/pipelines/update-dependencies-monitor.yml

@@ -24,6 +24,9 @@ stages:
         Update7:
           MajorMinorVersion: 7.$(monitor7MinorVersion)
           ChannelQuality: $(monitor7Quality)
+        Update8:
+          MajorMinorVersion: 8.$(monitor8MinorVersion)
+          ChannelQuality: $(monitor8Quality)
     steps:
     - pwsh: $(engPath)/Get-MonitorDropVersions.ps1 -Channel $(MajorMinorVersion)/$(ChannelQuality)
       displayName: Get Versions