build: support cmake 4.0 (#1214)

bump `cmake_minimum_required` to support cmake 4.0 build

```
    CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
      Compatibility with CMake < 3.5 has been removed from CMake.
```

Author: kkysen

c2rust-ast-exporter/src/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.4.3)
+cmake_minimum_required(VERSION 3.5)
 project(ASTExporter)
 
 #################################################

examples/json-c/repo/CMakeLists.txt

@@ -1,6 +1,6 @@
 #Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-cmake_minimum_required(VERSION 2.8.7)
+cmake_minimum_required(VERSION 3.5)
 cmake_policy(SET CMP0048 NEW)
 project(json-c VERSION 0.13.1)