Document dotnet-watch breaking change

Author: tmat

docs/core/compatibility/sdk/9.0/dotnet-watch.md

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+---
+title: "Breaking change: dotnet-watch requires disabling Hot Reload mode for projects targeting .NET 5 or older"
+description: Learn about a breaking change in the .NET 9 SDK where dotnet-watch requires disabling Hot Reload for projects targeting .NET 5 or older.
+ms.date: 07/09/2024
+---
+# Error reported for .NET 5 targets or older
+
+## Previous behavior
+
+Previously, `dotnet-watch` automatically disabled Hot Reload when used with projects targeting .NET 5 or older.
+
+## New behavior
+
+An error is reported when dotnet-watch is launched without `--no-hot-reload` on projects targeting .NET 5 or older:
+
+> Hot Reload based watching is only supported in .NET 6.0 or newer apps. 
+
+## Version introduced
+
+.NET 9 RC 1
+
+## Type of breaking change
+
+This change can affect development workflow.
+
+## Reason for change
+
+We have made significant changes in the internal architecture of the tool. Preserving behavior for out of support .NET versions did not warrant increasing complexity of the new implementation.
+
+## Recommended action
+
+Pass `--no-hot-reload` to `dotnet-watch` command line, or update your project to target `net6.0` or higher (`TargetFramework` property).
+
+## Affected APIs
+
+N/A
+