`String::append_utf*` functions missing.

### Godot version

4d1f26e1fd1fa46f2223fe0b6ac300744bf79b88

### godot-cpp version

master branch at 6a870949a5d38c62638af23dc13210f77154aa2c

### System information

Arch Linux

### Issue description

Attempting to use any of the `String::append_utf*` functions will generate a compiler error:

```
error: 'class godot::String' has no member named 'append_utf8'
```

These functions are equivalent to `String::parse_utf*` -- they were renamed in https://github.com/godotengine/godot/commit/ffa6ef220bdefe64b0d2ebb3c48c5dc54d52045b.

### Steps to reproduce

Attempt to use any of the `String::append_utf*`, compile, compiler error ensues.

### Minimal reproduction project

```cpp
String test_string = "testing";
test_string.append_utf8("compiler_error_here");
```