### 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(format != FORMAT_RGBA8);
	ERR_FAIL_COND(!data.size());
	int s = data.size();
	uint8_t *w = data.ptrw();
	for (int i = 0; i < s; i += 4) {
		uint8_t r = w[i];
		w[i] = w[i + 2]; 
		w[i + 2] = r; 
	}
}
Error Image::_load_from_buffer(const Vector<uint8_t> &p_array, ImageMemLoadFunc p_loader) {
	int buffer_size = p_array.size();
	ERR_FAIL_COND_V(buffer_size == 0, ERR_INVALID_PARAMETER);
	ERR_FAIL_COND_V(!p_loader, ERR_INVALID_PARAMETER);
	const uint8_t *r = p_array.ptr();
	Ref<Image> image = p_loader(r, buffer_size);
	ERR_FAIL_COND_V(!image.is_valid(), ERR_PARSE_ERROR);
	copy_internals_from(image);
	return OK;
}
void Image::average_4_uint8(uint8_t &p_out, const uint8_t &p_a, const uint8_t &p_b, const uint8_t &p_c, const uint8_t &p_d) {
	p_out = static_cast<uint8_t>((p_a + p_b + p_c + p_d + 2) >> 2);
}
void Image::average_4_float(float &p_out, const float &p_a, const float &p_b, const float &p_c, const float &p_d) {