### 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)

}
			for (int32_t dst_x = 0; dst_x < dst_width; dst_x++) {
				float pixel[CC] = { 0 };
				float weight = 0;
				for (int32_t target_y = start_y; target_y <= end_y; target_y++) {
					float lanczos_val = kernel[target_y - start_y];
					weight += lanczos_val;
					float *buffer_data = ((float *)buffer) + (target_y * dst_width + dst_x) * CC;
					for (uint32_t i = 0; i < CC; i++) {
						pixel[i] += buffer_data[i] * lanczos_val;
					}
				}
				T *dst_data = ((T *)p_dst) + (dst_y * dst_width + dst_x) * CC;
				for (uint32_t i = 0; i < CC; i++) {
					pixel[i] /= weight;
					if constexpr (sizeof(T) == 1) { 
						dst_data[i] = CLAMP(Math::fast_ftoi(pixel[i]), 0, 255);
					} else if constexpr (sizeof(T) == 2) { 
						dst_data[i] = Math::make_half_float(pixel[i]);
					} else { 
						dst_data[i] = pixel[i];
					}
				}
			}
		}
		memdelete_arr(kernel);
	} 
	memdelete_arr(buffer);
}
static void _overlay(const uint8_t *__restrict p_src, uint8_t *__restrict p_dst, float p_alpha, uint32_t p_width, uint32_t p_height, uint32_t p_pixel_size) {