### 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)

convert(Image::FORMAT_RGBA8);
	{
		int len = data.size() / 4;
		uint8_t *data_ptr = data.ptrw();
		for (int i = 0; i < len; i++) {
			data_ptr[(i << 2) + 3] = data_ptr[(i << 2) + 0]; 
			data_ptr[(i << 2) + 0] = data_ptr[(i << 2) + 1]; 
			data_ptr[(i << 2) + 2] = data_ptr[(i << 2) + 1];
		}
	}
	convert(Image::FORMAT_LA8);
}
Ref<Image> Image::rgbe_to_srgb() {
	if (data.size() == 0) {
		return Ref<Image>();
	}
	ERR_FAIL_COND_V(format != FORMAT_RGBE9995, Ref<Image>());
	Ref<Image> new_image = create_empty(width, height, false, Image::FORMAT_RGB8);
	for (int row = 0; row < height; row++) {
		for (int col = 0; col < width; col++) {
			new_image->set_pixel(col, row, get_pixel(col, row).linear_to_srgb());
		}
	}
	if (has_mipmaps()) {
		new_image->generate_mipmaps();
	}
	return new_image;
}
Ref<Image> Image::get_image_from_mipmap(int p_mipamp) const {