Fix pool used to generate manifests during release builds (PowerShell#392)

* use standard build pool

* use windows latest image

* run all windows builds in parallel

Author: TravisEz13

.vsts-ci/releaseStage.yml

@@ -40,4 +40,4 @@ stages:
         ACR: yes
         # failure rate is High in ACR for Windows and we have few images.
         # No reason to stress the system.
-        maxParallel: 1
+        maxParallel: 3

.vsts-ci/releasebuild.yml

@@ -32,11 +32,12 @@ stages:
       displayName: Create Preview Manifest Lists
       condition: succeededOrFailed()
       pool:
-        name: Package ES Standard Build
+        vmImage: windows-latest
         timeoutInMinutes: 30
 
       steps:
       - template: manifestSteps.yml
+
     - job: StableManifestPhase
       variables:
         releaseTag: $(stableReleaseTag)
@@ -46,11 +47,12 @@ stages:
 
       condition: succeededOrFailed()
       pool:
-        name: Package ES Standard Build
+        vmImage: windows-latest
         timeoutInMinutes: 30
 
       steps:
       - template: manifestSteps.yml
+
   - stage: GenerateTagsYaml
     dependsOn: []
     jobs: