### 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 to_x = MIN(width - 1, j + max_radius);
			int from_y = MAX(0, i - max_radius);
			int to_y = MIN(height - 1, i + max_radius);
			for (int k = from_y; k <= to_y; k++) {
				for (int l = from_x; l <= to_x; l++) {
					int dy = i - k;
					int dx = j - l;
					int dist = dy * dy + dx * dx;
					if (dist >= closest_dist) {
						continue;
					}
					const uint8_t *rp2 = &srcptr[(k * width + l) << 2];
					if (rp2[3] < alpha_threshold) {
						continue;
					}
					closest_dist = dist;
					closest_color[0] = rp2[0];
					closest_color[1] = rp2[1];
					closest_color[2] = rp2[2];
				}
			}
			if (closest_dist != max_dist) {
				wptr[0] = closest_color[0];
				wptr[1] = closest_color[1];
				wptr[2] = closest_color[2];
			}
		}
	}
}
String Image::get_format_name(Format p_format) {