Keep single-item tuple variants when upgrading from v1

[lorisleiva]

This PR corrects the upgrade guidance for v1 tuple variants holding a single item. The current guidance says upgrades should unwrap them into their item, except for structs, tuples and links. Instead, upgrades should keep them as tuples:

// v1
enumTupleVariantTypeNode('wait', tupleTypeNode([numberTypeNode('u64')]));
// v2
enumVariantTypeNode('wait', { data: tupleTypeNode([integerTypeNode('u64')]) });

This keeps the upgrade a faithful translation of the v1 IDL and leaves generated APIs unchanged. Authors can then simplify these variants to { data: integerTypeNode('u64') } themselves, and renderers can unwrap them before rendering if they wish.

Since the stable 2.0.0 changelog is built from the pre-release changesets, the correction amends the existing enum variant changeset in .changeset/pre/ rather than adding a new one. The stable changelog therefore states the correct guidance once. Its upgrade example is updated to keep the tuple too.