Bump stellar-xdr for SCSpecTypeUDTV2

[leighmcculloch]

Warning

Depends on unmerged and unreleased changes in:

• Add SCSpecTypeUDTV2 with a type id stellar-xdr#310
• Add borrowed Ref variants of generated types rs-stellar-xdr#560
• Add const XDR serialization on View types rs-stellar-xdr#562
• Regenerate with SCSpecTypeUDTV2 rs-stellar-xdr#564

A valid merge order for the whole stack:

1. Add SCSpecTypeUDTV2 with a type id stellar-xdr#310 — add SCSpecTypeUDTV2 to the contract spec XDR
2. Add borrowed Ref variants of generated types rs-stellar-xdr#560 — borrowed Ref variants of the generated types
3. Add const XDR serialization on View types rs-stellar-xdr#562 — const XDR serialization on those Ref types
4. Regenerate with SCSpecTypeUDTV2 rs-stellar-xdr#564 — regenerate with SCSpecTypeUDTV2
5. this PR
6. Encode contract spec XDR at const evaluation time rs-soroban-sdk#1965 — encode contract spec XDR at const evaluation time
7. Reference user-defined types by id rs-soroban-sdk#1966 — reference user-defined types by id

What

Move the pinned stellar-xdr rev forward to the regeneration that adds ScSpecTypeUdtv2 and its ScSpecTypeDef::UdtV2 variant.

Why

Nothing here reads or constructs the new variant — the host does not reference ScSpecTypeDef at all — so this has no effect on host behavior and needs no host code changes. It is here because rs-soroban-env pins stellar-xdr by rev, and that pin has to move to the rev carrying SCSpecTypeUDTV2 before an rs-soroban-env release can be cut against it. Landing it as its own change keeps the mechanical bump separate from the SDK work that actually uses the variant.

Known limitations

No test exercises the change, because nothing in the host touches the new variant; the coverage lives downstream in the SDK. Pins an unmerged rs-stellar-xdr branch, so check-git-rev-deps fails until that lands and the pin moves to a merged commit.