### 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(!_can_modify(format), "Cannot resize in compressed or custom image formats.");
	int w = next_power_of_2(width);
	int h = next_power_of_2(height);
	if (p_square) {
		w = h = MAX(w, h);
	}
	if (w == width && h == height) {
		if (!p_square || w == h) {
			return; 
		}
	}
	resize(w, h, p_interpolation);
}
void Image::resize(int p_width, int p_height, Interpolation p_interpolation) {
	ERR_FAIL_COND_MSG(data.size() == 0, "Cannot resize image before creating it, use set_data() first.");
	ERR_FAIL_COND_MSG(!_can_modify(format), "Cannot resize in compressed or custom image formats.");
	bool mipmap_aware = p_interpolation == INTERPOLATE_TRILINEAR ;
	ERR_FAIL_COND_MSG(p_width <= 0, "Image width must be greater than 0.");
	ERR_FAIL_COND_MSG(p_height <= 0, "Image height must be greater than 0.");
	ERR_FAIL_COND_MSG(p_width > MAX_WIDTH, "Image width cannot be greater than " + itos(MAX_WIDTH) + ".");
	ERR_FAIL_COND_MSG(p_height > MAX_HEIGHT, "Image height cannot be greater than " + itos(MAX_HEIGHT) + ".");
	ERR_FAIL_COND_MSG(p_width * p_height > MAX_PIXELS, "Too many pixels for image, maximum is " + itos(MAX_PIXELS));
	if (p_width == width && p_height == height) {
		return;
	}
	Image dst(p_width, p_height, false, format);
	
	Image dst2;
	int mip1 = 0;