### Godot version

2a0aef5f09

### System information

Windows 11, Ryzen 9 5950X, RTX 3090 (531.79) Vulkan, 64GB RAM

### Issue description

The issue occurs due to two things.
When textures/images are imported, the `fix_alpha_edges` / `fix_alpha_borders` function in the importer will modify the color values but leaves the alpha values unchanged.

Later, when using the `_convert` function in the `image.cpp` file or `Image::convert` to convert from `Image::FORMAT_RGBA` to `Image::FORMAT_L8` the following image results (raw on top, conversion on bottom).
![Clipboard - May 22, 2023 8_13 PM](https://github.com/godotengine/godot/assets/5207577/52f4597e-17b6-48c5-8fb4-d6d37012d8f4)

I, originally, thought that the `fix_alpha_edges` / `fix_alpha_borders` code was at fault but it turns out to not necessarily be the case.  I modified the `_convert` function in `image.cpp` to calculate grayscale values correctly and to consider the alpha channel or not and now I'm getting the expected output as shown here:
![Clipboard - May 23, 2023 7_16 AM](https://github.com/godotengine/godot/assets/5207577/73babb20-22bb-4f23-808f-5d49dce55f2d)


### Steps to reproduce

Import `Image::FORMAT_RGBA` image into any project.

Convert image in code by calling the `convert` function to `Image::FORMAT_L8`.

Screen shot of MRP here:
![image](https://github.com/godotengine/godot/assets/5207577/9d7d0078-5e41-45d1-b5c5-fc291b839fa9)


### Minimal reproduction project

Attached now
[Issue_77393.zip](https://github.com/godotengine/godot/files/11545149/Issue_77393.zip)

} else if (v >= 'A' && v <= 'F') {
								v = (v - 'A') + 10;
							} else if (v >= 'a' && v <= 'f') {
								v = (v - 'a') + 10;
							} else {
								break;
							}
							switch (i) {
								case 0:
									col_r = v << 4;
									break;
								case 1:
									col_r |= v;
									break;
								case 2:
									col_g = v << 4;
									break;
								case 3:
									col_g |= v;
									break;
								case 4:
									col_b = v << 4;
									break;
								case 5:
									col_b |= v;
									break;
							}
						}
						
						if (col_r == 255 && col_g == 0 && col_b == 255) {