### 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)

Dictionary result;
	result["max"] = INFINITY;
	result["mean"] = INFINITY;
	result["mean_squared"] = INFINITY;
	result["root_mean_squared"] = INFINITY;
	result["peak_snr"] = 0.0f;
	ERR_FAIL_NULL_V(p_compared_image, result);
	Error err = OK;
	Ref<Image> compared_image = duplicate(true);
	if (compared_image->is_compressed()) {
		err = compared_image->decompress();
	}
	ERR_FAIL_COND_V(err != OK, result);
	Ref<Image> source_image = p_compared_image->duplicate(true);
	if (source_image->is_compressed()) {
		err = source_image->decompress();
	}
	ERR_FAIL_COND_V(err != OK, result);
	ERR_FAIL_COND_V(err != OK, result);
	ERR_FAIL_COND_V_MSG((compared_image->get_format() >= Image::FORMAT_RH) && (compared_image->get_format() <= Image::FORMAT_RGBE9995), result, "Metrics on HDR images are not supported.");
	ERR_FAIL_COND_V_MSG((source_image->get_format() >= Image::FORMAT_RH) && (source_image->get_format() <= Image::FORMAT_RGBE9995), result, "Metrics on HDR images are not supported.");
	double image_metric_max, image_metric_mean, image_metric_mean_squared, image_metric_root_mean_squared, image_metric_peak_snr = 0.0;
	const bool average_component_error = true;
	const uint32_t w = MIN(compared_image->get_width(), source_image->get_width());
	const uint32_t h = MIN(compared_image->get_height(), source_image->get_height());
	
	double hist[256];