### 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 (w == minw && h == minh) {
				break;
			}
			w = MAX(minw, w >> 1);
			h = MAX(minh, h >> 1);
		}
		
		if (r_mm_width) {
			*r_mm_width = w;
		}
		if (r_mm_height) {
			*r_mm_height = h;
		}
		
		if (p_mipmaps >= 0 && mm == p_mipmaps) {
			break;
		}
		mm++;
	}
	r_mipmaps = mm;
	return size;
}
bool Image::_can_modify(Format p_format) const {
	return p_format <= FORMAT_RGBE9995;
}
template <class Component, int CC, bool renormalize,
		void (*average_func)(Component &, const Component &, const Component &, const Component &, const Component &),
		void (*renormalize_func)(Component *)>
static void _generate_po2_mipmap(const Component *p_src, Component *p_dst, uint32_t p_width, uint32_t p_height) {