JDK 16 RFR of JDK-8248605: Update --release 15 symbol information for JDK 15 build 31

[Joe Darcy]

Hello,

Please review the updates of the JDK 16 symbol information for JDK 15 b31:

     JDK-8248605: Update --release 15 symbol information for JDK 15 
build 31
     http://cr.openjdk.java.net/~darcy/8248605.0/

It is not actually clear to me why the script determined an update was 
needed. Jan, can you shed light here?

Thanks,

-Joe