### 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 (p_images[idx].is_null() || p_images[idx]->is_empty()) {
				return VALIDATE_3D_ERR_IMAGE_EMPTY;
			}
			if (p_images[idx]->get_format() != p_format) {
				return VALIDATE_3D_ERR_IMAGE_FORMAT_MISMATCH;
			}
			if (p_images[idx]->get_width() != w || p_images[idx]->get_height() != h) {
				return VALIDATE_3D_ERR_IMAGE_SIZE_MISMATCH;
			}
			if (p_images[idx]->has_mipmaps()) {
				return VALIDATE_3D_ERR_IMAGE_HAS_MIPMAPS;
			}
		}
		arr_ofs += d;
		if (!p_mipmaps) {
			break;
		}
		if (w == 1 && h == 1 && d == 1) {
			break;
		}
		w = MAX(1, w >> 1);
		h = MAX(1, h >> 1);
		d = MAX(1, d >> 1);
	}
	if (arr_ofs != p_images.size()) {
		return VALIDATE_3D_ERR_EXTRA_IMAGES;
	}
	return VALIDATE_3D_OK;
}
String Image::get_3d_image_validation_error_text(Image3DValidateError p_error) {