[ci] Fix reportgenerator installation error #3369

What I did
Fix pipeline error.
here is the error message:

/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202: Package dotnet-reportgenerator-globaltool 5.4.1 is not compatible with net7.0 (.NETCoreApp,Version=v7.0) / any. Package dotnet-reportgenerator-globaltool 5.4.1 supports:
/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202:   - net8.0 (.NETCoreApp,Version=v8.0) / any
/tmp/823d4a0e-75c7-4ca8-b487-06e1f38eae6d/restore.csproj : error NU1202:   - net9.0 (.NETCoreApp,Version=v9.0) / any

+ reportgenerator -reporttypes:Cobertura '-reports:tests/*coverage.xml' -targetdir:.
/agent/_work/_temp/057fa35b-9e5b-426a-bd88-2c5956cb8dcf.sh: line 2: reportgenerator: command not found
Why I did it

How I verified it
pipeline in this PR didn't shows that error message.

.azure-pipelines/test-docker-sonic-vs-template.yml

@@ -90,7 +90,7 @@ jobs:
         curl -sSL https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -
         sudo apt-add-repository https://packages.microsoft.com/ubuntu/20.04/prod
         sudo apt-get update
-        sudo apt-get install -y dotnet-sdk-7.0
+        sudo apt-get install -y dotnet-sdk-8.0
         sudo dotnet tool install dotnet-reportgenerator-globaltool --tool-path /usr/bin 2>&1 | tee log.log || grep 'already installed' log.log
         rm log.log
     displayName: "Install .NET CORE"