CI: Bump Node to 12.18.3 to match Electron 11

See: https://www.electronjs.org/releases/stable?version=11&page=6#11.0.0

Author: DeeDeeG

script/vsts/platforms/templates/preparation.yml

@@ -26,9 +26,9 @@ steps:
 
   - task: NodeTool@0
     inputs:
-      versionSpec: 12.16.3
+      versionSpec: 12.18.3
       force32bit: $(IsWinX86)
-    displayName: Install Node.js 12.16.3
+    displayName: Install Node.js 12.18.3
 
   - script: npm install --global [email protected]
     displayName: Update npm