docs: remove the 1.x breaking-changes callout from the README

The 2.0 migration warning is the first thing a new reader meets in the README,
where it is noise for anyone who never used 1.x. This deletes the callout and
the two horizontal rules that framed it, so the intro paragraph runs straight
into the feature list.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 984024677

Author: GWeale

README.md

@@ -35,16 +35,6 @@ simplify building, deploying, and orchestrating agent workflows, from simple
 tasks to complex systems. While optimized for Gemini, ADK is model-agnostic,
 deployment-agnostic, and compatible with other frameworks.
 
-______________________________________________________________________
-
-> **⚠️ BREAKING CHANGES FROM 1.x**
->
-> This release includes breaking changes to the agent API, event model, and
-> session schema. **Sessions generated by ADK 2.0 are readable by ADK 1.28+
-> (extra fields will be ignored), but are incompatible with older 1.x versions.**
-
-______________________________________________________________________
-
 ## ✨ Key Features
 
 - **Workflow Runtime**: A graph-based execution engine for composing