fix: shorten DevcontainersMerge friendlyName to satisfy tfx-cli 40-char limit

tfx-cli warns (and will eventually error) that friendlyName must be
<= 40 chars. The previous value was 43 chars. The new value matches
the sibling DevcontainersCi task's 'Devcontainers' prefix and uses
'Multi-Arch' (the term already used in the task description).

Author: tgenov

azdo-task/DevcontainersMerge/task.json

@@ -2,7 +2,7 @@
   "$schema": "https://raw.githubusercontent.com/microsoft/azure-pipelines-task-lib/master/tasks.schema.json",
   "id": "31bfde32-3b03-443f-9e5b-e902b849b3b8",
   "name": "DevcontainersMerge",
-  "friendlyName": "Dev Containers Merge Multi-Platform Images",
+  "friendlyName": "Devcontainers Merge Multi-Arch Images",
   "description": "Merge per-platform dev container images into a multi-arch manifest",
   "author": "Devcontainers",
   "helpMarkDown": "",