tools: fix CMake deprecation warnings (raspberrypi#1477)

Running a build with a newer CMake version results in a deprecation
warning from the tools subdirectory

    [13/108] Performing configure step for 'PioasmBuild'
    loading initial cache file /.../build/pico-sdk/src/rp2_common/pico_cyw43_driver/pioasm/tmp/PioasmBuild-cache-Release.cmake
    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.

Raise the minimum version requirement for the tools to the same version
that is required in the top-level CMakeLists.txt anyway.

tools/elf2uf2/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.13)
 project(elf2uf2)
 
 set(CMAKE_CXX_STANDARD 14)

tools/pioasm/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.4)
+cmake_minimum_required(VERSION 3.13)
 project(pioasm CXX)
 
 set(CMAKE_CXX_STANDARD 11)