v2.6.0: Fix for `MaskablePPO` with `SubprocVecEnv`, add Gymnasium v1.1 support

Breaking Changes:

• Upgraded to Stable-Baselines3 >= 2.6.0
• Renamed _dump_logs() to dump_logs()

New Features:

• Added support for Gymnasium v1.1.0

Bug Fixes:

• Fixed issues with SubprocVecEnv and MaskablePPO by using vec_env.has_attr() (pickling issues, mask function not present)

Full Changelog: v2.5.0...v2.6.0