### 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)

Ref<Image> image = memnew(Image(8, 4, false, Image::FORMAT_RGBA8));
	CHECK_MESSAGE(
			!image->is_empty(),
			"An image created with specified size and format should not be empty at first.");
	CHECK_MESSAGE(
			image->is_invisible(),
			"A newly created image should be invisible.");
	CHECK_MESSAGE(
			!image->is_compressed(),
			"A newly created image should not be compressed.");
	CHECK(!image->has_mipmaps());
	PackedByteArray image_data = image->get_data();
	for (int i = 0; i < image_data.size(); i++) {
		CHECK_MESSAGE(
				image_data[i] == 0,
				"An image created without data specified should have its data zeroed out.");
	}
	Ref<Image> image_copy = memnew(Image());
	CHECK_MESSAGE(
			image_copy->is_empty(),
			"An image created without any specified size and format be empty at first.");
	image_copy->copy_internals_from(image);
	CHECK_MESSAGE(
			image->get_data() == image_copy->get_data(),
			"Duplicated images should have the same data.");
	image_data = image->get_data();
	Ref<Image> image_from_data = memnew(Image(8, 4, false, Image::FORMAT_RGBA8, image_data));
	CHECK_MESSAGE(
			image->get_data() == image_from_data->get_data(),
			"An image created from data of another image should have the same data of the original image.");