docs: archive superseded version support proposals

Move MULTI_VERSION_SUPPORT.md and version-aware-mdl.md to archive/,
superseded by BSON_SCHEMA_REGISTRY_PROPOSAL.md which consolidates
the versioning strategy into a single comprehensive plan.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako