### 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)

data_bmp.resize(f_bmp->get_length() + 1);
	f_bmp->get_buffer(data_bmp.ptrw(), f_bmp->get_length());
	CHECK_MESSAGE(
			image_bmp->load_bmp_from_buffer(data_bmp) == OK,
			"The BMP image should load successfully.");
	
	Ref<Image> image_jpg = memnew(Image());
	Ref<FileAccess> f_jpg = FileAccess::open(TestUtils::get_data_path("images/icon.jpg"), FileAccess::READ, &err);
	REQUIRE(!f_jpg.is_null());
	PackedByteArray data_jpg;
	data_jpg.resize(f_jpg->get_length() + 1);
	f_jpg->get_buffer(data_jpg.ptrw(), f_jpg->get_length());
	CHECK_MESSAGE(
			image_jpg->load_jpg_from_buffer(data_jpg) == OK,
			"The JPG image should load successfully.");
	
	Ref<Image> image_webp = memnew(Image());
	Ref<FileAccess> f_webp = FileAccess::open(TestUtils::get_data_path("images/icon.webp"), FileAccess::READ, &err);
	REQUIRE(!f_webp.is_null());
	PackedByteArray data_webp;
	data_webp.resize(f_webp->get_length() + 1);
	f_webp->get_buffer(data_webp.ptrw(), f_webp->get_length());
	CHECK_MESSAGE(
			image_webp->load_webp_from_buffer(data_webp) == OK,
			"The WebP image should load successfully.");
	
	Ref<Image> image_png = memnew(Image());
	Ref<FileAccess> f_png = FileAccess::open(TestUtils::get_data_path("images/icon.png"), FileAccess::READ, &err);
	REQUIRE(!f_png.is_null());
	PackedByteArray data_png;