### 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)

"BPTC_RGBF",
	"BPTC_RGBFU",
	"ETC", 
	"ETC2_R11", 
	"ETC2_R11S", 
	"ETC2_RG11",
	"ETC2_RG11S",
	"ETC2_RGB8",
	"ETC2_RGBA8",
	"ETC2_RGB8A1",
	"ETC2_RA_AS_RG",
	"FORMAT_DXT5_RA_AS_RG",
	"ASTC_4x4",
	"ASTC_4x4_HDR",
	"ASTC_8x8",
	"ASTC_8x8_HDR",
};
SavePNGFunc Image::save_png_func = nullptr;
SaveJPGFunc Image::save_jpg_func = nullptr;
SaveEXRFunc Image::save_exr_func = nullptr;
SavePNGBufferFunc Image::save_png_buffer_func = nullptr;
SaveEXRBufferFunc Image::save_exr_buffer_func = nullptr;
SaveJPGBufferFunc Image::save_jpg_buffer_func = nullptr;
SaveWebPFunc Image::save_webp_func = nullptr;
SaveWebPBufferFunc Image::save_webp_buffer_func = nullptr;
void Image::_put_pixelb(int p_x, int p_y, uint32_t p_pixel_size, uint8_t *p_data, const uint8_t *p_pixel) {
	uint32_t ofs = (p_y * width + p_x) * p_pixel_size;
	memcpy(p_data + ofs, p_pixel, p_pixel_size);
}
void Image::_get_pixelb(int p_x, int p_y, uint32_t p_pixel_size, const uint8_t *p_data, uint8_t *p_pixel) {