MNT: delete tools.cached_property

[Gui-FernandesBR]

Is your feature request related to a problem? Please describe.

Since we no longer support python 3.7, we don't need to keep our own implementation of the cached_property decorator.

Describe the solution you'd like

• [x] Delete the cached_property definition inside the tools.py module.
• [x] Adjust all the imports throughout the rocketpy module

In order to avoid breaking changes, I think we could import the cached_property in the tools.py module so it doesn't break when we do from rocketpy.tools import cached_property