chore: bump Rust backend to 0.8.3

[shinaoka]

Summary

• bump rust_backend_version to 0.8.3
• update the crates.io fallback references in the docs to sparse-ir-capi 0.8.3

Verification

• env -u SPARSEIR_RUST_BACKEND_DIR julia --project=. -e 'using Pkg; Pkg.build("SparseIR"); Pkg.test()' from a temp copy without a sibling sparse-ir-rs checkout\n- result: 657/657 tests passed against published sparse-ir-capi 0.8.3

[terasakisatoshi]

I've found that this PR is no longer needed. I'll close this PR

See #135 #133