book: Update instructions to v4_12

Author: Hofer-Julian

book/src/project_setup.md

@@ -16,12 +16,14 @@ pkg-config --modversion gtk4
 ```
 
 Use this information to add the [gtk4 crate](https://crates.io/crates/gtk4) to your dependencies in `Cargo.toml`.
-At the time of this writing the newest version is `4.8`.
+At the time of this writing the newest version is `4.12`.
 
 ```
-cargo add gtk4 --rename gtk --features v4_8
+cargo add gtk4 --rename gtk --features v4_12
 ```
 
+By specifying this feature you opt-in to API that was added with minor releases of GTK 4.
+
 Now, you can run your application by executing:
 ```
 cargo run