### 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)

if (w == h) { 
			for (int y = 0; y < h / 2; y++) {
				for (int x = 0; x < (w + 1) / 2; x++) {
					int current = y * w + x;
					memcpy(single_pixel_buffer, data_ptr + current * pixel_size, pixel_size);
					for (int i = 0; i < 3; i++) {
						int prev = PREV_INDEX_IN_CYCLE(current);
						memcpy(data_ptr + current * pixel_size, data_ptr + prev * pixel_size, pixel_size);
						current = prev;
					}
					memcpy(data_ptr + current * pixel_size, single_pixel_buffer, pixel_size);
				}
			}
		} else { 
			int permuted_pixels_count = 0;
			for (int i = 0; i < size; i++) {
				int prev = PREV_INDEX_IN_CYCLE(i);
				if (prev == i) {
					
					permuted_pixels_count++;
					continue;
				}
				
				
				
				
				while (prev > i) {
					prev = PREV_INDEX_IN_CYCLE(prev);
				}
				if (prev < i) {