chore: update ci image pools for brownout

.ci/pipeline.yml

@@ -14,7 +14,7 @@ jobs:
 
 - job: Linux
   pool:
-    vmImage: 'ubuntu-18.04'
+    vmImage: 'ubuntu-latest'
   steps:
   - template: common-validation.yml
   variables:
@@ -22,7 +22,7 @@ jobs:
 
 - job: LinuxMinspec
   pool:
-    vmImage: 'ubuntu-18.04'
+    vmImage: 'ubuntu-latest'
   steps:
   - template: common-validation.yml
   variables: