xdr: update go monorepo XDR

[tsachiherman]

What ?

We need to update the go monorepo XDR to the latest XDR that supports AuthNext.
That would allow the dependent soroban-rpc to take advantage of it.

PR in go monorepo : https://github.com/stellar/go/pull/4756

this issue is part of the authNext efforts, which are covered in #346

[paulbellamy]

XDR to update is in this PR stellar/stellar-xdr#65