### 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)

for (int i = 0; i < width; i++) {
			if (!(get_pixel(i, j).a > 0)) {
				continue;
			}
			if (i > maxx) {
				maxx = i;
			}
			if (j > maxy) {
				maxy = j;
			}
			if (i < minx) {
				minx = i;
			}
			if (j < miny) {
				miny = j;
			}
		}
	}
	if (maxx == -1) {
		return Rect2i();
	} else {
		return Rect2i(minx, miny, maxx - minx + 1, maxy - miny + 1);
	}
}
Ref<Image> Image::get_region(const Rect2i &p_region) const {
	Ref<Image> img = memnew(Image(p_region.size.x, p_region.size.y, mipmaps, format));
	img->blit_rect(Ref<Image>((Image *)this), p_region, Point2i(0, 0));
	return img;
}
void Image::_get_clipped_src_and_dest_rects(const Ref<Image> &p_src, const Rect2i &p_src_rect, const Point2i &p_dest, Rect2i &r_clipped_src_rect, Rect2i &r_clipped_dest_rect) const {