### 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)

} break;
					case ROUGHNESS_CHANNEL_L: {
						c.r = roughness;
						c.g = roughness;
						c.b = roughness;
					} break;
					case ROUGHNESS_CHANNEL_A: {
						c.a = roughness;
					} break;
				}
				_set_color_at_ofs(ptr, pixel_ofs, c);
			}
		}
#if 0
		{
			int size = get_mipmap_byte_size(i);
			print_line("size for mimpap " + itos(i) + ": " + itos(size));
			Vector<uint8_t> imgdata;
			imgdata.resize(size);
			uint8_t* wr = imgdata.ptrw();
			memcpy(wr.ptr(), ptr, size);
			wr = uint8_t*();
			Ref<Image> im = Image::create_from_data(w, h, false, format, imgdata);
			im->save_png("res:mipmap_" + itos(i) + ".png");
		}
#endif
	}
	return OK;
}
void Image::clear_mipmaps() {