ci: pin down Ubuntu versions (#8388)

For a couple weeks now there has been a warning on the Azure Pipelines
web UI that says `ubuntu-latest` is in the process of switching from
18.04 to 20.04. I am not aware of any specific issue this would cause
for our particular workflows, but I don't like my dependencies changing
from under me.

CHANGELOG_BEGIN
CHANGELOG_END

azure-pipelines.yml

@@ -276,7 +276,7 @@ jobs:
 - job: compat_versions_pr_trigger_daily
   dependsOn: compat_versions_pr
   pool:
-    vmImage: "ubuntu-latest"
+    vmImage: "ubuntu-18.04"
   variables:
     branch: $[ dependencies.compat_versions_pr.outputs['out.branch'] ]
   steps:

ci/cron/tuesday.yml

@@ -16,7 +16,7 @@ jobs:
 - job: announce_rotation
   timeoutInMinutes: 60
   pool:
-    vmImage: ubuntu-latest
+    vmImage: ubuntu-18.04
   steps:
   - checkout: self
     persistCredentials: true

ci/cron/wednesday.yml

@@ -16,7 +16,7 @@ jobs:
 - job: open_release_pr
   timeoutInMinutes: 60
   pool:
-    vmImage: ubuntu-latest
+    vmImage: ubuntu-18.04
   steps:
   - checkout: self
     persistCredentials: true