Update XDR reference

[sreuland]

What problem does your feature solve?

there is a newer version of xdr available for Protocol 20, soroban:
stellar-xdr:7015193879e9da7cb70d889a49eecf494fefe7e1

for reference, same update to xdr being done on mono repo here:
https://github.com/stellar/go/pull/5023/files#diff-76ed074a9305c04054cdebb9e9aad2d818052b07091de1f20cad0bbac34ffb52

What would you like to see?

the xdr generated classes in java are updated to this version of xdr and any compile time issues that may arise are resolved.

What alternatives are there?

[sreuland]

@overcat , might be good to hold on doing this xdr version until we know it's confirmed, there's another adjustment about to land - stellar/stellar-xdr#137

[sreuland]

@overcat , I think it's getting closer to stable for xdr version for next release, this latest xdr version may be the candidate to generate from, the js sdk is sitting on this version for now also:

https://github.com/stellar/stellar-xdr/tree/65afa63b7f52c898143ebbe9541ef91fcf290ade

[overcat]

Implemented in #530.