Set version of new identifiers after 3.1.3 release to 3.1.6

include/SDL3/SDL_system.h

@@ -595,7 +595,7 @@ typedef enum SDL_Sandbox
  * \returns the application sandbox environment or SDL_SANDBOX_NONE if the
  *          application is not running in a sandbox environment.
  *
- * \since This function is available since SDL 3.1.4.
+ * \since This function is available since SDL 3.1.6.
  */
 extern SDL_DECLSPEC SDL_Sandbox SDLCALL SDL_GetSandbox(void);