### 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)

CHECK_MESSAGE(
			image->get_size() == Vector2(4, 4),
			"get_size() should return the correct size after cropping.");
	image->set_pixel(0, 0, Color(1, 1, 1, 1));
	
	for (int i = 0; i < 5; i++) {
		Ref<Image> image_resized = memnew(Image());
		image_resized->copy_internals_from(image);
		Image::Interpolation interpolation = static_cast<Image::Interpolation>(i);
		image_resized->resize(8, 8, interpolation);
		CHECK_MESSAGE(
				image_resized->get_size() == Vector2(8, 8),
				"get_size() should return the correct size after resizing.");
		CHECK_MESSAGE(
				image_resized->get_pixel(1, 1).a > 0,
				"Resizing an image should also affect its content.");
	}
	
	image->shrink_x2();
	CHECK_MESSAGE(
			image->get_size() == Vector2(2, 2),
			"get_size() should return the correct size after shrink_x2().");
	
	Ref<Image> image_po_2 = memnew(Image(14, 28, false, Image::FORMAT_RGBA8));
	image_po_2->resize_to_po2();
	CHECK_MESSAGE(
			image_po_2->get_size() == Vector2(16, 32),
			"get_size() should return the correct size after resize_to_po2().");
}
TEST_CASE("[Image] Modifying pixels of an image") {