Stellar releases Soroban Rust SDK v28 with spec shaking v2 and easier data migration

• —Spec shaking v2 is now always on, stripping unused spec entries from the Wasm binary
• —New struct fields read as None from old data; removed fields are ignored on unpack
• —Void and Option::None event fields are omitted from published map events
• —Builds require stellar-cli v25.2.0 or newer; the export argument is removed