ReplacementTransform and z_index issue

Nobody has claimed this yet.

## Description of bug / unexpected behavior

When I use the `ReplacementTransform(source_mobject, target_mobject)` function to transform a mobject, I find that `target_mobject.z_index` is unchanged, but the `target_mobject` will show as that `target_mobject.z_index` is changed to `source_mobject.z_index`.

Please see the code and gif below, I expect that the blue circle is above the red circle because the z_index of the blue circle is greater, but the result show that the blue circle is below the red circle.

## Expected behavior

The blue circle is above the red circle.

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *

class zIndexBug(Scene):
def construct(self):
circle = Circle(color=RED, fill_opacity=1).move_to(UP*0.5)
circle.set_z_index(3)
circle2 = Circle(color=BLUE, fill_opacity=1)
circle2.set_z_index(6)

circle3 = Circle(color=GREEN, fill_opacity=1).move_to(LEFT*3)
circle3.set_z_index(1)

self.play(FadeIn(circle3), FadeIn(circle))
self.play(ReplacementTransform(circle3, circle2))

```

## Additional media files

Images/GIFs


## Logs
Terminal output

```
PASTE HERE OR PROVIDE LINK TO https://pastebin.com/ OR SIMILAR
```

## System specifications

System Details

- OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
```
PASTE HERE
```

LaTeX details

+ LaTeX distribution (e.g. TeX Live 2020):
+ Installed LaTeX packages:

FFMPEG

Output of `ffmpeg -version`:

```
PASTE HERE
```

## Additional comments

Start by running the provided Scene reproduction and inspect ReplacementTransform's handling of source_mobject and target_mobject z_index during the animation. Done means the blue target circle renders above the red circle according to its higher z_index, while the reported target_mobject.z_index behavior is consistent.

Written by the indexing model from the issue text.