Update Maze environments

[rodrigodelazcano]

Description

This PR makes the following updates to the maze environments (pointmaze and antmaze):

• Default reset_target to False.
• When there are no goals ("g") or reset ("r") cells provided in the maze_map structure, the environment will select these cells at random from the empty cells ("0") when reset.
• Allow serialization of the reset_target argument with EzPickle

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes