### 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)

float lanczos_val = kernel[target_x - start_x];
					weight += lanczos_val;
					const T *__restrict src_data = ((const T *)p_src) + (buffer_y * src_width + target_x) * CC;
					for (uint32_t i = 0; i < CC; i++) {
						if constexpr (sizeof(T) == 2) { 
							pixel[i] += Math::half_to_float(src_data[i]) * lanczos_val;
						} else {
							pixel[i] += src_data[i] * lanczos_val;
						}
					}
				}
				float *dst_data = ((float *)buffer) + (buffer_y * dst_width + buffer_x) * CC;
				for (uint32_t i = 0; i < CC; i++) {
					dst_data[i] = pixel[i] / weight; 
				}
			}
		}
		memdelete_arr(kernel);
	} 
	{ 
		float y_scale = float(src_height) / float(dst_height);
		float scale_factor = MAX(y_scale, 1);
		int32_t half_kernel = LANCZOS_TYPE * scale_factor;
		float *kernel = memnew_arr(float, half_kernel * 2);
		for (int32_t dst_y = 0; dst_y < dst_height; dst_y++) {
			float buffer_y = (dst_y + 0.5f) * y_scale;
			int32_t start_y = MAX(0, int32_t(buffer_y) - half_kernel + 1);
			int32_t end_y = MIN(src_height - 1, int32_t(buffer_y) + half_kernel);
			for (int32_t target_y = start_y; target_y <= end_y; target_y++) {
				kernel[target_y - start_y] = _lanczos((target_y + 0.5f - buffer_y) / scale_factor);