Update to preview 7

[mmitche]

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[ericstj]

Code change and versions look good.

Test failure seems unrelated, filed dotnet/dnceng#3807 and reran failing leg to confirm it's transient. Other PRs are not hitting it.

Also hit by @riarenas here: #14968 (comment)