ci: Use latest ubuntu image for mainline/patreon step 1

.ci/yuzu-mainline-step1.yml

@@ -1,6 +1,9 @@
 trigger:
 - master
 
+pool:
+  vmImage: 'ubuntu-latest'
+
 stages:
 - stage: merge
   displayName: 'merge'

.ci/yuzu-patreon-step1.yml

@@ -1,6 +1,9 @@
 trigger:
 - master
 
+pool:
+  vmImage: 'ubuntu-latest'
+
 stages:
 - stage: merge
   displayName: 'merge'