switch azure ubuntu image from 16.04 to 20.04 (#1335)

Author: kandersolar

azure-pipelines.yml

@@ -9,7 +9,7 @@ jobs:
 - template: ci/azure/posix.yml
   parameters:
     name: Test_bare_Linux
-    vmImage: ubuntu-16.04
+    vmImage: ubuntu-20.04
 
 
 - template: ci/azure/posix.yml
@@ -21,7 +21,7 @@ jobs:
 - template: ci/azure/conda_linux.yml
   parameters:
     name: Test_conda_linux
-    vmImage: ubuntu-16.04
+    vmImage: ubuntu-20.04
 
 
 - template: ci/azure/conda_windows.yml