### 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 (data.size() == 0) {
		return;
	}
	if (format != FORMAT_RGBA8) {
		return; 
	}
	Vector<uint8_t> dcopy = data;
	const uint8_t *srcptr = dcopy.ptr();
	uint8_t *data_ptr = data.ptrw();
	const int max_radius = 4;
	const int alpha_threshold = 20;
	const int max_dist = 0x7FFFFFFF;
	for (int i = 0; i < height; i++) {
		for (int j = 0; j < width; j++) {
			const uint8_t *rptr = &srcptr[(i * width + j) * 4];
			uint8_t *wptr = &data_ptr[(i * width + j) * 4];
			if (rptr[3] >= alpha_threshold) {
				continue;
			}
			int closest_dist = max_dist;
			uint8_t closest_color[3];
			int from_x = MAX(0, j - max_radius);