[release/3.1] Stop building on Ubuntu 18.04

- use 20.04 instead

nit: Remove Ubuntu version from one CI job display name

Author: dougbu

.azure/pipelines/ci.yml

@@ -662,7 +662,7 @@ stages:
     parameters:
       condition: ne(variables['SkipTests'], 'true')
       jobName: Linux_Test
-      jobDisplayName: "Test: Ubuntu 18.04 x64"
+      jobDisplayName: "Test: Ubuntu x64"
       agentOs: Linux
       isTestingJob: true
       buildArgs: --all --test "/p:RunTemplateTests=false"  $(_InternalRuntimeDownloadArgs)
@@ -700,7 +700,7 @@ stages:
     displayName: 'Test: Linux Source Build'
     container: centos:7
     pool:
-      vmImage: 'ubuntu-18.04'
+      vmImage: 'ubuntu-20.04'
     variables:
       LC_ALL: 'en_US.UTF-8'
       LANG: 'en_US.UTF-8'

.azure/pipelines/jobs/default-build.yml

@@ -91,7 +91,7 @@ jobs:
       ${{ if eq(parameters.agentOs, 'macOS') }}:
         vmImage: macOS-11
       ${{ if eq(parameters.agentOs, 'Linux') }}:
-        vmImage: ubuntu-18.04
+        vmImage: ubuntu-20.04
       ${{ if eq(parameters.agentOs, 'Windows') }}:
         ${{ if eq(variables['System.TeamProject'], 'public') }}:
           name: NetCore-Public