### 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)

status = DONE;
				}
			} break;
			default: {
			}
		}
		line++;
	}
}
#define DETECT_ALPHA_MAX_THRESHOLD 254
#define DETECT_ALPHA_MIN_THRESHOLD 2
#define DETECT_ALPHA(m_value)                          \
	{                                                  \
		uint8_t value = m_value;                       \
		if (value < DETECT_ALPHA_MIN_THRESHOLD)        \
			bit = true;                                \
		else if (value < DETECT_ALPHA_MAX_THRESHOLD) { \
			detected = true;                           \
			break;                                     \
		}                                              \
	}
#define DETECT_NON_ALPHA(m_value) \
	{                             \
		uint8_t value = m_value;  \
		if (value > 0) {          \
			detected = true;      \
			break;                \
		}                         \
	}
bool Image::is_invisible() const {