ci: Use ubuntu-latest vmImage where applicable

Not specifying the vmImage defaults to ubuntu-16.04, which will be deprecated soon and is experiencing brownouts.

.ci/templates/merge-private.yml

@@ -1,6 +1,8 @@
 jobs:
 - job: merge
   displayName: 'pull requests'
+  pool:
+    vmImage: 'ubuntu-latest'
   steps:
   - checkout: self
     submodules: recursive
@@ -24,6 +26,8 @@ jobs:
 - job: upload_source
   displayName: 'upload'
   dependsOn: merge
+  pool:
+    vmImage: 'ubuntu-latest'
   steps:
   - template: ./sync-source.yml
     parameters:

.ci/templates/merge.yml

@@ -1,6 +1,8 @@
 jobs:
 - job: merge
   displayName: 'pull requests'
+  pool:
+    vmImage: 'ubuntu-latest'
   steps:
   - checkout: self
     submodules: recursive
@@ -23,6 +25,8 @@ jobs:
 - job: upload_source
   displayName: 'upload'
   dependsOn: merge
+  pool:
+    vmImage: 'ubuntu-latest'
   steps:
   - template: ./sync-source.yml
     parameters:

.ci/yuzu-mainline-step2.yml

@@ -66,5 +66,7 @@ stages:
   jobs:
   - job: github
     displayName: 'github'
+    pool:
+      vmImage: ubuntu-latest
     steps:
     - template: ./templates/release-github.yml

.ci/yuzu-patreon-step2.yml

@@ -29,5 +29,7 @@ stages:
   jobs:
     - job: release
       displayName: 'source'
+      pool:
+        vmImage: 'ubuntu-latest'
       steps:
         - template: ./templates/release-private-tag.yml