### 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)

if (save_exr_func == nullptr) {
		return ERR_UNAVAILABLE;
	}
	return save_exr_func(p_path, Ref<Image>((Image *)this), p_grayscale);
}
Vector<uint8_t> Image::save_exr_to_buffer(bool p_grayscale) const {
	if (save_exr_buffer_func == nullptr) {
		return Vector<uint8_t>();
	}
	return save_exr_buffer_func(Ref<Image>((Image *)this), p_grayscale);
}
Error Image::save_webp(const String &p_path, const bool p_lossy, const float p_quality) const {
	if (save_webp_func == nullptr) {
		return ERR_UNAVAILABLE;
	}
	ERR_FAIL_COND_V_MSG(p_lossy && !(0.0f <= p_quality && p_quality <= 1.0f), ERR_INVALID_PARAMETER, "The WebP lossy quality was set to " + rtos(p_quality) + ", which is not valid. WebP lossy quality must be between 0.0 and 1.0 (inclusive).");
	return save_webp_func(p_path, Ref<Image>((Image *)this), p_lossy, p_quality);
}
Vector<uint8_t> Image::save_webp_to_buffer(const bool p_lossy, const float p_quality) const {
	if (save_webp_buffer_func == nullptr) {
		return Vector<uint8_t>();
	}
	ERR_FAIL_COND_V_MSG(p_lossy && !(0.0f <= p_quality && p_quality <= 1.0f), Vector<uint8_t>(), "The WebP lossy quality was set to " + rtos(p_quality) + ", which is not valid. WebP lossy quality must be between 0.0 and 1.0 (inclusive).");
	return save_webp_buffer_func(Ref<Image>((Image *)this), p_lossy, p_quality);
}
int Image::get_image_data_size(int p_width, int p_height, Format p_format, bool p_mipmaps) {
	int mm;
	return _get_dst_image_size(p_width, p_height, p_format, mm, p_mipmaps ? -1 : 0);
}
int Image::get_image_required_mipmaps(int p_width, int p_height, Format p_format) {