Consider usage of a rust-toolchain file

[data-sync-user]

We might want to consider using a rust-toolchain file to standardize what version is used. I think it still makes sense to specify rust versions in e.g. docker images (see RUST_VER) as it should make setup of these images quicker because it’s explicit, and doesn’t require such images to potentially adjust to the rust-toolchain file it runs against.

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