### 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)

int ofs, size, w, h;
	get_mipmap_offset_size_and_dimensions(p_mipamp, ofs, size, w, h);
	Vector<uint8_t> new_data;
	new_data.resize(size);
	{
		uint8_t *wr = new_data.ptrw();
		const uint8_t *rd = data.ptr();
		memcpy(wr, rd + ofs, size);
	}
	Ref<Image> image;
	image.instantiate();
	image->width = w;
	image->height = h;
	image->format = format;
	image->data = new_data;
	image->mipmaps = false;
	return image;
}
void Image::bump_map_to_normal_map(float bump_scale) {
	ERR_FAIL_COND(!_can_modify(format));
	clear_mipmaps();
	convert(Image::FORMAT_RF);
	Vector<uint8_t> result_image; 
	result_image.resize(width * height * 4);
	{
		const uint8_t *rp = data.ptr();
		uint8_t *wp = result_image.ptrw();
		ERR_FAIL_COND(!rp);