Prevents future migrations from hitting error

[cameronvoell]

Fixes #1339

currently on main branch cargo run --bin update-schema --features update-schema from xmtp_mls directory creates the following code diff:
https://github.com/xmtp/libxmtp/compare/cv/test-migration-on-main?expand=1#diff-bd2907561f23f5c88f95cebd28c7a4bb56585017a04c75fbce3eff3997863788

[insipx]

Since its not the first time we're running into this, could be worth adding the extra derives on StoredGroup to avoid the ugly error

#[derive(Selectable) + #[diesel(check_for_backend(crate::storage::sqlite::Sqlite))]