Update README to reflect Test Engine deprecation (#679)

* Update README to reflect Test Engine deprecation

Updated README to indicate deprecation of Test Engine and recommend alternatives.

* Revise deprecation notice in README

Updated deprecation notice with a removal date of November 1, 2026.

Author: brflood

README.md

@@ -2,7 +2,8 @@
 
 [![CI Build](https://github.com/microsoft/PowerApps-TestEngine/actions/workflows/build-test.yml/badge.svg)](https://github.com/microsoft/PowerApps-TestEngine/actions/workflows/build-test.yml)
 
-> This is currently an preview project.
+> [!IMPORTANT]
+> Test Engine is **deprecated** and this repository will be removed on November 1, 2026. Use [Power Platform Playwright samples](https://learn.microsoft.com/power-platform/developer/playwright-samples/overview) for test automation capabilities in Power Platform and Dynamics 365 services. See [Migrate from Power Apps Test Engine](https://learn.microsoft.com/power-platform/developer/playwright-samples/migration-guide)
 
 ## Overview