### 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)

rects.push_back(Rect2i(0, img_height, 12, 12));
		rects.push_back(Rect2i(img_width + 1, img_height + 2, 12, 12));
		rects.push_back(Rect2i(1, 1, 1, 1));
		rects.push_back(Rect2i(0, 1, 2, 3));
		rects.push_back(Rect2i(-5, 0, img_width + 10, 2));
		rects.push_back(Rect2i(0, -5, 2, img_height + 10));
		rects.push_back(Rect2i(-1, -1, img_width + 1, img_height + 1));
		for (const Rect2i &rect : rects) {
			Ref<Image> img = memnew(Image(img_width, img_height, false, Image::FORMAT_RGBA8));
			CHECK_NOTHROW_MESSAGE(
					img->fill_rect(rect, Color(1, 1, 1, 1)),
					"fill_rect() shouldn't throw for any rect.");
			for (int y = 0; y < img->get_height(); y++) {
				for (int x = 0; x < img->get_width(); x++) {
					if (rect.abs().has_point(Point2(x, y))) {
						CHECK_MESSAGE(
								img->get_pixel(x, y).is_equal_approx(Color(1, 1, 1, 1)),
								"fill_rect() should colorize all image pixels within rect bounds.");
					} else {
						CHECK_MESSAGE(
								!img->get_pixel(x, y).is_equal_approx(Color(1, 1, 1, 1)),
								"fill_rect() shouldn't colorize any image pixel out of rect bounds.");
					}
				}
			}
		}
	}
	
	image->blend_rect(image2, Rect2i(Vector2i(0, 0), image2->get_size()), Vector2i(0, 0));
	CHECK_MESSAGE(