Document API key naming deprecations

Signed-off-by: Mathias L. Baumann <mathias.baumann@frequenz.com>

Author: Marenz

RELEASE_NOTES.md

@@ -8,6 +8,11 @@ Raise `ValueError` when `start_time` passed to `create()` is neither a `datetime
 
 <!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
 
+- `DispatchApiClient` now uses `key` as the canonical authentication parameter;
+  `auth_key` remains supported but is deprecated. Likewise, the CLI now prefers
+  `--api-key` / `DISPATCH_API_KEY` over the deprecated `--auth-key` /
+  `DISPATCH_API_AUTH_KEY` names.
+
 ## New Features
 
 <!-- Here goes the main new features and examples or instructions on how to use them -->