### 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)

ClassDB::bind_method(D_METHOD("get_region", "region"), &Image::get_region);
	ClassDB::bind_method(D_METHOD("copy_from", "src"), &Image::copy_internals_from);
	ClassDB::bind_method(D_METHOD("_set_data", "data"), &Image::_set_data);
	ClassDB::bind_method(D_METHOD("_get_data"), &Image::_get_data);
	ClassDB::bind_method(D_METHOD("get_pixelv", "point"), &Image::get_pixelv);
	ClassDB::bind_method(D_METHOD("get_pixel", "x", "y"), &Image::get_pixel);
	ClassDB::bind_method(D_METHOD("set_pixelv", "point", "color"), &Image::set_pixelv);
	ClassDB::bind_method(D_METHOD("set_pixel", "x", "y", "color"), &Image::set_pixel);
	ClassDB::bind_method(D_METHOD("adjust_bcs", "brightness", "contrast", "saturation"), &Image::adjust_bcs);
	ClassDB::bind_method(D_METHOD("load_png_from_buffer", "buffer"), &Image::load_png_from_buffer);
	ClassDB::bind_method(D_METHOD("load_jpg_from_buffer", "buffer"), &Image::load_jpg_from_buffer);
	ClassDB::bind_method(D_METHOD("load_webp_from_buffer", "buffer"), &Image::load_webp_from_buffer);
	ClassDB::bind_method(D_METHOD("load_tga_from_buffer", "buffer"), &Image::load_tga_from_buffer);
	ClassDB::bind_method(D_METHOD("load_bmp_from_buffer", "buffer"), &Image::load_bmp_from_buffer);
	ADD_PROPERTY(PropertyInfo(Variant::DICTIONARY, "data", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_STORAGE), "_set_data", "_get_data");
	BIND_CONSTANT(MAX_WIDTH);
	BIND_CONSTANT(MAX_HEIGHT);
	BIND_ENUM_CONSTANT(FORMAT_L8); 
	BIND_ENUM_CONSTANT(FORMAT_LA8); 
	BIND_ENUM_CONSTANT(FORMAT_R8);
	BIND_ENUM_CONSTANT(FORMAT_RG8);
	BIND_ENUM_CONSTANT(FORMAT_RGB8);
	BIND_ENUM_CONSTANT(FORMAT_RGBA8);
	BIND_ENUM_CONSTANT(FORMAT_RGBA4444);
	BIND_ENUM_CONSTANT(FORMAT_RGB565);
	BIND_ENUM_CONSTANT(FORMAT_RF); 
	BIND_ENUM_CONSTANT(FORMAT_RGF);
	BIND_ENUM_CONSTANT(FORMAT_RGBF);
	BIND_ENUM_CONSTANT(FORMAT_RGBAF);
	BIND_ENUM_CONSTANT(FORMAT_RH);