COMP: Upgrade Ubuntu version at Azure pipelines CI from 18.0.4 to 20.0.4

The image of Ubuntu 18.0.4 still has Python 3.6, which has reached "the end-of-life phase of its release cycle", as of 2021-12-2, according to https://peps.python.org/pep-0494/

Ubuntu 20.04.4 has installed Python3 3.8.10 out of the box, according to https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md

Aims to solve a Python error from TransformixTest (under construction), saying:

> TypeError: __init__() got an unexpected keyword argument 'capture_output'

Testing/CI/Azure/ci.yml

@@ -60,10 +60,10 @@ jobs:
   - bash: ctest -C Release -VV -j 2 -E "elastix_run_example_COMPARE_IM|elastix_run_3DCT_lung.MI.bspline.ASGD.001_COMPARE_TP"
     displayName: 'CTest Elastix'
     workingDirectory: $(ELASTIX_BINARY_DIR)
-- job: Ubuntu1804
+- job: Ubuntu
   timeoutInMinutes: 0
   pool:
-    vmImage: 'ubuntu-18.04'
+    vmImage: 'ubuntu-20.04'
   strategy:
     matrix:
       ITKv5: