Migration: detect v4 install and point users to v5

When `func --version` (or any command) detects a v4 install alongside
v5, surface a one-line migration pointer.

## Definition of Done

- [ ] Detection heuristic defined
- [ ] Pointer message wired into `func --version` (and any other safe
surface)
- [ ] Behavior covered by tests

Start by tracing the implementation behind `func --version` and locating the command's test entry point; the issue names no file or test path. Define the v4/v5 detection heuristic, wire the one-line pointer into safe command surfaces, and add tests covering detection and non-detection.

Written by the indexing model from the issue text.