Remove Node 6 and Node 10 runners from Microsoft-hosted agents

After a series of changes described on the roadmap, we will finally remove the end-of-life versions of Node from Microsoft-hosted agents. Once we make this change, tasks that target Node 6 or Node 10 will fail on Microsoft-hosted agents. To reduce the impact this may have on you, we will:

• Give ample notice in blog posts and release notes.
• Expose warnings in pipeline logs in the months before removal when tasks target Node 6 or 10.
• Conduct brownouts (where using tasks that target Node 6 or 10 generates errors) to catch your attention and fix problematic pipelines before the final removal.
• Introduce a feature to re-target a pipeline's Node 6 or Node 10 tasks to Node 16 (at your own risk).