### 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)

uint32_t dst_w = MAX(p_width >> 1, 1u);
	uint32_t dst_h = MAX(p_height >> 1, 1u);
	int right_step = (p_width == 1) ? 0 : CC;
	int down_step = (p_height == 1) ? 0 : (p_width * CC);
	for (uint32_t i = 0; i < dst_h; i++) {
		const Component *rup_ptr = &p_src[i * 2 * down_step];
		const Component *rdown_ptr = rup_ptr + down_step;
		Component *dst_ptr = &p_dst[i * dst_w * CC];
		uint32_t count = dst_w;
		while (count) {
			count--;
			for (int j = 0; j < CC; j++) {
				average_func(dst_ptr[j], rup_ptr[j], rup_ptr[j + right_step], rdown_ptr[j], rdown_ptr[j + right_step]);
			}
			if (renormalize) {
				renormalize_func(dst_ptr);
			}
			dst_ptr += CC;
			rup_ptr += right_step * 2;
			rdown_ptr += right_step * 2;
		}
	}
}
void Image::shrink_x2() {