Deprecate V3 publishing in main

Now that the vast majority of repos have onboarded to Arcade V4 publishing (tracked in https://github.com/dotnet/arcade/issues/16697), we should plan the deprecation and eventual removal of V3 publishing infrastructure in `main`.

### Scope
- Determine the criteria/gate for declaring V3 publishing deprecated (e.g. all in-scope `main` repos onboarded to V4).
- Add deprecation warnings when V3 publishing paths are used.
- Define the removal timeline and communicate to repo owners still on V3.
- Track and drive the remaining stragglers to V4 before removal.

### Context
Migrated from personal work-tracking item W134. Follow-up to the V4 publishing rollout.

Start by reviewing the V4 publishing rollout tracked in issue 16697 and inventorying the V3 publishing paths in main. The issue provides no file or test entry point; completion would require agreed deprecation criteria, warnings, a removal timeline, and follow-up with remaining V3 repositories.

Written by the indexing model from the issue text.