Ubuntu 18.04 queues

[michaelgsharp]

With Ubuntu 16.04 queues no longer being supported in Azure devops actions/runner-images#3287, this PR switches all the Ubuntu queues (not the docker images yet) from 16.04 to 18.04.

[codecov]

Codecov Report

Merging #5970 (fda7fd7) into main (f696661) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5970      +/-   ##
==========================================
- Coverage   68.24%   68.23%   -0.01%     
==========================================
  Files        1142     1143       +1     
  Lines      242803   242869      +66     
  Branches    25385    25388       +3     
==========================================
+ Hits       165704   165725      +21     
- Misses      70407    70447      +40     
- Partials     6692     6697       +5     

Flag	Coverage Δ
Debug	68.23% <ø> (-0.01%)	⬇️
production	62.89% <ø> (-0.02%)	⬇️
test	88.62% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0.00%> (-20.52%)	⬇️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	78.74% <0.00%> (-6.30%)	⬇️
...c/Microsoft.ML.Data/Prediction/PredictionEngine.cs	87.83% <0.00%> (-4.81%)	⬇️
src/Microsoft.ML.TimeSeries/PredictionEngine.cs	76.96% <0.00%> (-3.63%)	⬇️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	62.25% <0.00%> (-3.32%)	⬇️
src/Microsoft.ML.OnnxTransformer/OnnxUtils.cs	84.48% <0.00%> (-2.15%)	⬇️
.../Microsoft.ML.Data/Model/ModelOperationsCatalog.cs	53.60% <0.00%> (-1.78%)	⬇️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	84.74% <0.00%> (-0.85%)	⬇️
src/Microsoft.ML.Core/Data/Repository.cs	79.50% <0.00%> (-0.28%)	⬇️
src/Microsoft.ML.OnnxTransformer/OnnxCatalog.cs	100.00% <0.00%> (ø)
... and 5 more

[michaelgsharp]

Turns out I could get away with just changing from pool name to vmImage and no other changes were required, at least for now. Once the docker image is ready I will create a new PR to get those in.