### 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)

}
	if (format > FORMAT_RGBE9995 || p_new_format > FORMAT_RGBE9995) {
		ERR_FAIL_MSG("Cannot convert to <-> from compressed formats. Use compress() and decompress() instead.");
	} else if (format > FORMAT_RGBA8 || p_new_format > FORMAT_RGBA8) {
		
		Image new_img(width, height, false, p_new_format);
		for (int i = 0; i < width; i++) {
			for (int j = 0; j < height; j++) {
				new_img.set_pixel(i, j, get_pixel(i, j));
			}
		}
		if (has_mipmaps()) {
			new_img.generate_mipmaps();
		}
		_copy_internals_from(new_img);
		return;
	}
	Image new_img(width, height, false, p_new_format);
	const uint8_t *rptr = data.ptr();
	uint8_t *wptr = new_img.data.ptrw();
	int conversion_type = format | p_new_format << 8;
	switch (conversion_type) {
		case FORMAT_L8 | (FORMAT_LA8 << 8):
			_convert<1, false, 1, true, true, true>(width, height, rptr, wptr);
			break;
		case FORMAT_L8 | (FORMAT_R8 << 8):
			_convert<1, false, 1, false, true, false>(width, height, rptr, wptr);
			break;
		case FORMAT_L8 | (FORMAT_RG8 << 8):
			_convert<1, false, 2, false, true, false>(width, height, rptr, wptr);