### 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 j = 0; j < dest_rect.size.x; j++) {
			int src_x = src_rect.position.x + j;
			int src_y = src_rect.position.y + i;
			
			
			
			if (msk->get_pixel(src_x, src_y).a != 0) {
				int dst_x = dest_rect.position.x + j;
				int dst_y = dest_rect.position.y + i;
				Color sc = img->get_pixel(src_x, src_y);
				if (sc.a != 0) {
					Color dc = get_pixel(dst_x, dst_y);
					dc = dc.blend(sc);
					set_pixel(dst_x, dst_y, dc);
				}
			}
		}
	}
}
void Image::_repeat_pixel_over_subsequent_memory(uint8_t *p_pixel, int p_pixel_size, int p_count) {
	int offset = 1;
	for (int stride = 1; offset + stride <= p_count; stride *= 2) {
		memcpy(p_pixel + offset * p_pixel_size, p_pixel, stride * p_pixel_size);
		offset += stride;
	}
	if (offset < p_count) {
		memcpy(p_pixel + offset * p_pixel_size, p_pixel, (p_count - offset) * p_pixel_size);
	}
}
void Image::fill(const Color &p_color) {