deps: bump burn 0.20 → 0.21, uuid 1.11 → 1.23

Resolves RUSTSEC-2026-0097 (rand 0.10.0 unsoundness): burn 0.21 brings a
transitive rand bump to 0.10.1, the patched version. uuid 1.23.1 follows
naturally. Cargo audit drops from 7 → 6 advisories; the remaining 6 are
all transitive within the burn ML stack (bincode, core2, paste,
rustls-pemfile, lru) and require upstream fixes. Workspace builds clean;
verisim-tensor tests still pass.

https://claude.ai/code/session_01GeiWCLLoZmPdnjMMcy2buu

Author: claude