Update frequenz-api-electricity-trading requirement

Updates the requirements on [frequenz-api-electricity-trading](https://github.com/frequenz-floss/frequenz-api-electricity-trading) to permit the latest version.
- [Release notes](https://github.com/frequenz-floss/frequenz-api-electricity-trading/releases)
- [Changelog](https://github.com/frequenz-floss/frequenz-api-electricity-trading/blob/v0.x.x/RELEASE_NOTES.md)
- [Commits](frequenz-floss/frequenz-api-electricity-trading@v0.5.0...v0.6.1)

---
updated-dependencies:
- dependency-name: frequenz-api-electricity-trading
  dependency-version: 0.6.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -45,7 +45,7 @@ dependencies = [
   "frequenz-channels >= 1.6.1, < 2",
   "frequenz-client-base >= 0.9.0, < 0.10.0",
   "frequenz-client-common >= 0.1.0, < 0.4.0",
-  "frequenz-api-electricity-trading >= 0.5.0, < 0.6.0",
+  "frequenz-api-electricity-trading >= 0.5.0, < 0.7.0",
   "protobuf >= 5.29.2, < 6",
 ]
 dynamic = ["version"]