docs: add migration guide for upgrading from v1 to v2

This PR adds UPGRADING.md, a comprehensive migration guide for users
upgrading from the v1 Replicate Python SDK to v2.

The guide covers:
- Client initialization changes (Replicate vs Client, bearer_token vs api_token)
- Prediction lifecycle changes (resource methods vs instance methods)
- Async support differences (AsyncReplicate client vs async_ methods)
- Error handling improvements (granular exception types)
- Pagination enhancements (auto-pagination)
- Models and versions API changes (keyword arguments)
- File handling differences
- New features in v2 (models listing, better types, HTTP customization)
- Removed features (instance methods, positional arguments)
- Migration strategy recommendations

Addresses DP-684

Author: zeke