Update to new rust_decimal when rkyv is removed

See https://github.com/paupino/rust-decimal/issues/818 and related discussions. Once `rkyv` is removed—however it is removed—update `rust_decimal` to remove `rkyv` from the lockfile and remove the exemption from `.cargo/audit.toml`.

Read the linked rust-decimal issue and inspect Cargo.lock and .cargo/audit.toml to confirm how rkyv is removed. Once that prerequisite is complete, update rust_decimal, remove rkyv from the lockfile, and remove its audit exemption; done means the dependency and exemption are both absent.

Written by the indexing model from the issue text.