Update c++ version

Author: yhmtsai

index.html

@@ -67,7 +67,7 @@ <h1 class="display-3"><img style="width: 35%" class="img-fluid" src="https://gin
 		img/ginkgo-1-horizontal.png"></h1>
 		<p>Ginkgo is a high-performance linear algebra library for
 			manycore systems, with a focus on solution of sparse linear systems.
-			It is implemented using modern C++ (you will need at least a C++14
+			It is implemented using modern C++ (you will need at least a C++17
 			compliant compiler to build it), with GPU kernels implemented in
 			CUDA (for NVIDIA devices), HIP (for AMD devices) and SYCL/DPC++ (for
 			Intel devices and other supported hardware).</p>