Change Feature.ArtifactV4Upgrade default to true

[jsoref]

• Split from Remove node_modules and compile typescript into minified scripts #2578 per Remove node_modules and compile typescript into minified scripts #2578 (comment)

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows. -- This change is not backwards compatible with existing workflows (if they use the debug artifact feature which I've never seen outside of this repository)
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[jsoref]

Note: I've left a file that tests the old v3 behavior. It's clearly called legacy, and I saw no particular reason to remove it yet as I'm not removing the code that it validates and someone who has access to GHES could conceivably want to test it.

This PR gave me enough insight that I could probably write some code to let that workflow skip doing real work unless it was running under GHES, although the lazy test would be to check the server URL as I did in my initial work.