### 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)

oy = (double)y * yfac - 0.5f;
		oy1 = (int)oy;
		dy = oy - (double)oy1;
		for (uint32_t x = 0; x < p_dst_width; x++) {
			
			ox = (double)x * xfac - 0.5f;
			ox1 = (int)ox;
			dx = ox - (double)ox1;
			
			T *__restrict dst = ((T *)p_dst) + (y * p_dst_width + x) * CC;
			double color[CC];
			for (int i = 0; i < CC; i++) {
				color[i] = 0;
			}
			for (int n = -1; n < 3; n++) {
				
				[[maybe_unused]] double k1 = _bicubic_interp_kernel(dy - (double)n);
				oy2 = oy1 + n;
				if (oy2 < 0) {
					oy2 = 0;
				}
				if (oy2 > ymax) {
					oy2 = ymax;
				}
				for (int m = -1; m < 3; m++) {
					
					[[maybe_unused]] double k2 = k1 * _bicubic_interp_kernel((double)m - dx);
					ox2 = ox1 + m;
					if (ox2 < 0) {
						ox2 = 0;