Switch to C++-17

This is expected to fail in the CI (at least on devtoolset6), but let's
see if the devtoolset7 passes.

This needed a new enough CMake which is now available in EPEL.

Change-Id: I8381c14c064fa2fc67d20fb81641ad851af3b9fa
diff --git a/CMakeLists.txt b/CMakeLists.txt
index cc97c23..3811457 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,6 @@
 project(netconf-cli LANGUAGES CXX)
 cmake_minimum_required(VERSION 3.0)
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include(GNUInstallDirs)