### 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)

bool used_mipmaps = has_mipmaps();
	if (used_mipmaps) {
		clear_mipmaps();
	}
	for (int y = 0; y < height; y++) {
		for (int x = 0; x < width; x++) {
			Color c = get_pixel(x, y);
			Vector3 v(c.r * 2.0 - 1.0, c.g * 2.0 - 1.0, c.b * 2.0 - 1.0);
			v.normalize();
			c.r = v.x * 0.5 + 0.5;
			c.g = v.y * 0.5 + 0.5;
			c.b = v.z * 0.5 + 0.5;
			set_pixel(x, y, c);
		}
	}
	if (used_mipmaps) {
		generate_mipmaps(true);
	}
}
Error Image::generate_mipmaps(bool p_renormalize) {
	ERR_FAIL_COND_V_MSG(!_can_modify(format), ERR_UNAVAILABLE, "Cannot generate mipmaps in compressed or custom image formats.");
	ERR_FAIL_COND_V_MSG(format == FORMAT_RGBA4444, ERR_UNAVAILABLE, "Cannot generate mipmaps from RGBA4444 format.");
	ERR_FAIL_COND_V_MSG(width == 0 || height == 0, ERR_UNCONFIGURED, "Cannot generate mipmaps with width or height equal to 0.");
	int mmcount;
	int size = _get_dst_image_size(width, height, format, mmcount);
	data.resize(size);
	uint8_t *wp = data.ptrw();
	int prev_ofs = 0;
	int prev_h = height;