Document breaking change to config get command output

Previously, the `config dump` CLI command returned the effective configuration, including both the values explicitly set
via the configuration file and environment variables as well as the values provided by defaults.

It was changed to only return the explicitly set configuration data. The breaking change was not documented at that time.

Author: per1234

docs/UPGRADING.md

@@ -530,6 +530,15 @@ breaking changes as needed.
   { "sketch_path": "/tmp/my_sketch" }
   ```
 
+### `config dump` no longer returns default configuration values
+
+Previously, the `config dump` CLI command returned the effective configuration, including both the values explicitly set
+via the configuration file and environment variables as well as the values provided by defaults.
+
+It now only returns the explicitly set configuration data.
+
+Use `config get <configuration key>` to get the effective value of the configuration
+
 ### The gRPC response `cc.arduino.cli.commands.v1.CompileResponse` has been changed.
 
 The `CompilerResponse` message has been refactored to made explicit which fields are intended for streaming the build