### 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)

const T *src = ((const T *)p_src);
					T *dst = ((T *)p_dst);
					float p00 = Math::half_to_float(src[y_ofs_up + src_xofs_left + l]);
					float p10 = Math::half_to_float(src[y_ofs_up + src_xofs_right + l]);
					float p01 = Math::half_to_float(src[y_ofs_down + src_xofs_left + l]);
					float p11 = Math::half_to_float(src[y_ofs_down + src_xofs_right + l]);
					float interp_up = p00 + (p10 - p00) * xofs_frac;
					float interp_down = p01 + (p11 - p01) * xofs_frac;
					float interp = interp_up + ((interp_down - interp_up) * yofs_frac);
					dst[i * p_dst_width * CC + j * CC + l] = Math::make_half_float(interp);
				} else if constexpr (sizeof(T) == 4) { 
					float xofs_frac = float(src_xofs_frac) / (1 << FRAC_BITS);
					float yofs_frac = float(src_yofs_frac) / (1 << FRAC_BITS);
					const T *src = ((const T *)p_src);
					T *dst = ((T *)p_dst);
					float p00 = src[y_ofs_up + src_xofs_left + l];
					float p10 = src[y_ofs_up + src_xofs_right + l];
					float p01 = src[y_ofs_down + src_xofs_left + l];
					float p11 = src[y_ofs_down + src_xofs_right + l];
					float interp_up = p00 + (p10 - p00) * xofs_frac;
					float interp_down = p01 + (p11 - p01) * xofs_frac;
					float interp = interp_up + ((interp_down - interp_up) * yofs_frac);
					dst[i * p_dst_width * CC + j * CC + l] = interp;
				}
			}
		}
	}
}
template <int CC, class T>
static void _scale_nearest(const uint8_t *__restrict p_src, uint8_t *__restrict p_dst, uint32_t p_src_width, uint32_t p_src_height, uint32_t p_dst_width, uint32_t p_dst_height) {