build: set minimum required CMake to 3.12

Avoids:
```bash
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.
```

Matches the minimum version used by miniupnpc:
https://github.com/miniupnp/miniupnp/blob/e98515ac5e68a7fc1134f9bc8d746e52ccd21790/miniupnpc/CMakeLists.txt#L1.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.12 FATAL_ERROR)
 
 if(POLICY CMP0048)
 	cmake_policy(SET CMP0048 NEW)