### 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)

}
TEST_CASE("[Image] Saving and loading") {
	Ref<Image> image = memnew(Image(4, 4, false, Image::FORMAT_RGBA8));
	const String save_path_png = OS::get_singleton()->get_cache_path().path_join("image.png");
	const String save_path_exr = OS::get_singleton()->get_cache_path().path_join("image.exr");
	
	Error err;
	err = image->save_png(save_path_png);
	CHECK_MESSAGE(
			err == OK,
			"The image should be saved successfully as a .png file.");
	
	err = image->save_exr(save_path_exr, false);
	CHECK_MESSAGE(
			err == OK,
			"The image should be saved successfully as an .exr file.");
	
	Ref<Image> image_load = memnew(Image());
	err = image_load->load(save_path_png);
	CHECK_MESSAGE(
			err == OK,
			"The image should load successfully using load().");
	CHECK_MESSAGE(
			image->get_data() == image_load->get_data(),
			"The loaded image should have the same data as the one that got saved.");
	
	Ref<Image> image_bmp = memnew(Image());
	Ref<FileAccess> f_bmp = FileAccess::open(TestUtils::get_data_path("images/icon.bmp"), FileAccess::READ, &err);
	REQUIRE(!f_bmp.is_null());
	PackedByteArray data_bmp;