fnsince: update \since policy for documentation.

Everything in SDL3 up to the ABI lock is reported as available since 3.1.3.
Everything else will be reported as since 3.2.0 (what will be the first
official release).

Also ran a Perl script over the headers to change everything to 3.1.3 that
wasn't an API function, since fnsince.pl can't manage those. If there's a
macro or datatype that has snuck in that needs to be 3.2.0 instead, we'll
have to manually fix it up, but it shouldn't be a big deal in any case.

Reference PR #11304.

include/SDL3/SDL_scancode.h

@@ -42,7 +42,7 @@
  * The values in this enumeration are based on the USB usage page standard:
  * https://usb.org/sites/default/files/hut1_5.pdf
  *
- * \since This enum is available since SDL 3.0.0.
+ * \since This enum is available since SDL 3.1.3.
  */
 typedef enum SDL_Scancode
 {