[SYCL][Doc] Update C++ standard support in GSG (#3478)

* DPC++ compiler builds applications as C++17 apps and higher

Author: dm-vodopyanov

sycl/doc/GetStartedGuide.md

@@ -640,7 +640,8 @@ which contains all the symbols required.
 ## C++ standard
 
 * DPC++ runtime and headers require C++14 at least.
-* DPC++ compiler is building apps as C++17 apps by default.
+* DPC++ compiler builds apps as C++17 apps by default. Higher versions of
+  standard are supported as well.
 
 ## Known Issues and Limitations