chore(deps): Update stable-baselines3 requirement

Updates the requirements on [stable-baselines3](https://github.com/DLR-RM/stable-baselines3) to permit the latest version.
- [Release notes](https://github.com/DLR-RM/stable-baselines3/releases)
- [Commits](https://github.com/DLR-RM/stable-baselines3/compare/v2.8.0...v2.9.0)

---
updated-dependencies:
- dependency-name: stable-baselines3
  dependency-version: 2.9.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
           pytest test/backtesting/ -v --tb=short
 
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v7
+        uses: codecov/codecov-action@v6
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: false

requirements/rl.txt

@@ -8,7 +8,7 @@
 # Only install if you want to use full PPO/A2C/SAC training.
 
 # Core RL library
-stable-baselines3[extra]>=2.8.0
+stable-baselines3[extra]>=2.9.0
 
 # Gymnasium environment (OpenAI Gym successor)
 gymnasium>=0.29.1