Simplify upgrade section in the readme as currently it is talking about breaking changes in v1.2 (and there already was v2 with additional changes)

[kavalerov]

As you can see here https://github.com/ably/ably-python?tab=readme-ov-file#breaking-api-changes-in-version-120 there is a whole section in the Readme related to changes in the version 1.2. However, there already was another major version release since then.

This can be a bit confusing, and as we list all breaking changes in Migration Guide. This PR suggests a simpler readme section that just points to the migration guide.