Update lockfiles (#3821)

## Description
This PR updates lockfiles by running:
```
./gradlew aws:sdk:generateAllLockfiles -Paws-sdk-rust-path=/Users/awsaito/src/aws-sdk-rust
```
However, due to [the minicbor
issue](https://github.com/smithy-lang/smithy-rs/pull/3818), we still pin
it to 0.24.2 by running the following on `rust-runtime/Cargo.lock` and
`aws/sdk/Cargo.lock`:
```
RUSTFLAGS="--cfg aws_sdk_unstable" cargo update -p minicbor --precise 0.24.2
```

The rest of the changes handle miscellaneous scenarios:
- **Updated nightly version**: Upgraded to `nightly-2024-03-15` to
address a [compatibility
issue](https://github.com/serde-rs/serde/issues/2770) introduced by the
updated serde library.
- **Cleaned up use statements**: Removed redundant use statements, which
were flagged as warnings by the new nightly version.

## Testing
Tests in CI

----

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