### 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)

ERR_FAIL_COND_MSG(p_src.is_null(), "It's not a reference to a valid Image object.");
	int dsize = data.size();
	int srcdsize = p_src->data.size();
	ERR_FAIL_COND(dsize == 0);
	ERR_FAIL_COND(srcdsize == 0);
	ERR_FAIL_COND(format != p_src->format);
	ERR_FAIL_COND_MSG(!_can_modify(format), "Cannot blit_rect in compressed or custom image formats.");
	Rect2i src_rect;
	Rect2i dest_rect;
	_get_clipped_src_and_dest_rects(p_src, p_src_rect, p_dest, src_rect, dest_rect);
	if (!src_rect.has_area() || !dest_rect.has_area()) {
		return;
	}