Revert to ubuntu-16.04.

This reverts part of 7d9a1de.

Author: bgrainger

azure-pipelines.yml

@@ -6,7 +6,7 @@ jobs:
 - job: linux_build
   displayName: 'Linux Build'
   pool:
-    vmimage: 'ubuntu-latest'
+    vmimage: 'ubuntu-16.04'
   steps:
   - template: '.ci/build-steps.yml'
 
@@ -113,7 +113,7 @@ jobs:
   dependsOn: linux_build
   displayName: 'Linux Unit Tests'
   pool:
-    vmimage: 'ubuntu-latest'
+    vmimage: 'ubuntu-16.04'
   steps:
   - template: '.ci/mysqlconnector-tests-steps.yml'
 
@@ -129,7 +129,7 @@ jobs:
   dependsOn: linux_build
   displayName: 'Linux Tests'
   pool:
-    vmimage: 'ubuntu-latest'
+    vmimage: 'ubuntu-16.04'
   strategy:
     matrix:
       'MySQL 5.6':