### 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)

_repeat_pixel_over_subsequent_memory(rect_first_pixel_ptr, pixel_size, r.size.x);
		for (int y = 1; y < r.size.y; y++) {
			memcpy(rect_first_pixel_ptr + y * width * pixel_size, rect_first_pixel_ptr, r.size.x * pixel_size);
		}
	}
}
ImageMemLoadFunc Image::_png_mem_loader_func = nullptr;
ImageMemLoadFunc Image::_jpg_mem_loader_func = nullptr;
ImageMemLoadFunc Image::_webp_mem_loader_func = nullptr;
ImageMemLoadFunc Image::_tga_mem_loader_func = nullptr;
ImageMemLoadFunc Image::_bmp_mem_loader_func = nullptr;
void (*Image::_image_compress_bc_func)(Image *, Image::UsedChannels) = nullptr;
void (*Image::_image_compress_bptc_func)(Image *, Image::UsedChannels) = nullptr;
void (*Image::_image_compress_etc1_func)(Image *) = nullptr;
void (*Image::_image_compress_etc2_func)(Image *, Image::UsedChannels) = nullptr;
void (*Image::_image_compress_astc_func)(Image *, Image::ASTCFormat) = nullptr;
void (*Image::_image_decompress_bc)(Image *) = nullptr;
void (*Image::_image_decompress_bptc)(Image *) = nullptr;
void (*Image::_image_decompress_etc1)(Image *) = nullptr;
void (*Image::_image_decompress_etc2)(Image *) = nullptr;
void (*Image::_image_decompress_astc)(Image *) = nullptr;
Vector<uint8_t> (*Image::webp_lossy_packer)(const Ref<Image> &, float) = nullptr;
Vector<uint8_t> (*Image::webp_lossless_packer)(const Ref<Image> &) = nullptr;
Ref<Image> (*Image::webp_unpacker)(const Vector<uint8_t> &) = nullptr;
Vector<uint8_t> (*Image::png_packer)(const Ref<Image> &) = nullptr;
Ref<Image> (*Image::png_unpacker)(const Vector<uint8_t> &) = nullptr;
Vector<uint8_t> (*Image::basis_universal_packer)(const Ref<Image> &, Image::UsedChannels) = nullptr;
Ref<Image> (*Image::basis_universal_unpacker)(const Vector<uint8_t> &) = nullptr;
Ref<Image> (*Image::basis_universal_unpacker_ptr)(const uint8_t *, int) = nullptr;
void Image::_set_data(const Dictionary &p_data) {