Set version of new identifiers after 3.1.3 release to 3.1.6

include/SDL3/SDL_stdinc.h

@@ -2564,7 +2564,7 @@ extern SDL_DECLSPEC Uint32 SDLCALL SDL_StepUTF8(const char **pstr, size_t *pslen
  *
  * \threadsafety It is safe to call this function from any thread.
  *
- * \since This function is available since SDL 3.1.4.
+ * \since This function is available since SDL 3.1.6.
  */
 extern SDL_DECLSPEC Uint32 SDLCALL SDL_StepBackUTF8(const char *start, const char **pstr);