using `color` instead of `fill_color` with MathTeX for node labels

[Schefflera-Arboricola]

fixes https://github.com/ManimCommunity/manim/issues/4500

Overview: What does this pull request change?

Replaced fill_color= with color= in all MathTex() calls throughout the codebase

Motivation and Explanation: Why and how do your changes improve the library?

Before: MathTex("x^2", fill_color=BLACK) --> white text
After: MathTex("x^2", color=BLACK) --> black text

white node labels on white node --> labels were not visible.

Links to added or changed documentation pages

https://manimce--4501.org.readthedocs.build/en/4501/reference/manim.mobject.graph.Graph.html -- here you can see the labels showing up in the examples (LabeledModifiedGraph example)

Further Information and Comments

NA

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested