### 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)

return width;
}
int Image::get_height() const {
	return height;
}
Size2i Image::get_size() const {
	return Size2i(width, height);
}
bool Image::has_mipmaps() const {
	return mipmaps;
}
int Image::get_mipmap_count() const {
	if (mipmaps) {
		return get_image_required_mipmaps(width, height, format);
	} else {
		return 0;
	}
}
template <uint32_t read_bytes, bool read_alpha, uint32_t write_bytes, bool write_alpha, bool read_gray, bool write_gray>
static void _convert(int p_width, int p_height, const uint8_t *p_src, uint8_t *p_dst) {
	uint32_t max_bytes = MAX(read_bytes, write_bytes);
	for (int y = 0; y < p_height; y++) {
		for (int x = 0; x < p_width; x++) {
			const uint8_t *rofs = &p_src[((y * p_width) + x) * (read_bytes + (read_alpha ? 1 : 0))];
			uint8_t *wofs = &p_dst[((y * p_width) + x) * (write_bytes + (write_alpha ? 1 : 0))];
			uint8_t rgba[4] = { 0, 0, 0, 255 };
			if constexpr (read_gray) {
				rgba[0] = rofs[0];