### 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)

unsigned char *write_ptr = wp;
		float *read_ptr = (float *)rp;
		for (int ty = 0; ty < height; ty++) {
			int py = ty + 1;
			if (py >= height) {
				py -= height;
			}
			for (int tx = 0; tx < width; tx++) {
				int px = tx + 1;
				if (px >= width) {
					px -= width;
				}
				float here = read_ptr[ty * width + tx];
				float to_right = read_ptr[ty * width + px];
				float above = read_ptr[py * width + tx];
				Vector3 up = Vector3(0, 1, (here - above) * bump_scale);
				Vector3 across = Vector3(1, 0, (to_right - here) * bump_scale);
				Vector3 normal = across.cross(up);
				normal.normalize();
				write_ptr[((ty * width + tx) << 2) + 0] = (127.5 + normal.x * 127.5);
				write_ptr[((ty * width + tx) << 2) + 1] = (127.5 + normal.y * 127.5);
				write_ptr[((ty * width + tx) << 2) + 2] = (127.5 + normal.z * 127.5);
				write_ptr[((ty * width + tx) << 2) + 3] = 255;
			}
		}
	}
	format = FORMAT_RGBA8;
	data = result_image;
}
void Image::srgb_to_linear() {