The second parameter should not be named with a fixed literal string "dt"

Usually function parameters are dummy variables that can be named anything.
Forcing us to use `dt` just adds unnecessary confusion to users.

https://github.com/ManimCommunity/manim/blob/117563e1f01adafe675f011ad425a257753ec9f4/manim/mobject/mobject.py#L975

Start at manim/mobject/mobject.py around line 975 and inspect the second parameter's definition and uses. Determine an appropriate non-fixed parameter name, update the affected references, and run the relevant tests to confirm the API still behaves correctly without requiring the literal "dt".

Written by the indexing model from the issue text.