Rust Notes app: upgrade scylla driver to 0.8.1

[tarzanek]

cargo fmt applied cargo clippy fixed

[tarzanek]

@guy9 @Gor027 addressed some of the comments from Gor

as follow up for https://github.com/scylladb/scylla-code-samples/pull/192

[tarzanek]

the rust rest server was tested with Scylla 5.0