Reduce duplicated dependencies on some HTTP-/TLS-related crates. (#10417)

## Description

Reduce duplicated HTTP/TLS-related dependencies pulled in by the AWS SDK
crates.

Two changes to `app/Cargo.toml`:

1. **`aws-sdk-sts`**: Add `default-features = false` with explicit
`default-https-client` and `rt-tokio` features. The default features
included `rustls`, which activated `aws-smithy-runtime/tls-rustls`,
pulling in the legacy `hyper-rustls 0.24` + `rustls 0.21` path alongside
the modern `rustls 0.23` we actually use. This eliminates the `rustls
0.21.12` dependency entirely.

2. **`aws-config`**: Bump from `1.8.12` to `1.8.16`. The older version
depended on `aws-smithy-http ^0.62.6`, while the updated
`aws-smithy-runtime` needed `^0.63.6`, causing both versions to coexist.
The bump unifies everything on `aws-smithy-http 0.63.x`.

## Testing

- [x] `cargo check -p warp` passes
- [x] Verified `rustls 0.21` is no longer in the dependency tree
- [x] Verified `aws-smithy-http` is unified on a single version (0.63.x)
- [x] Dependency-only change — no code changes

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: vorporeal