Refactor(Rust/Hello-world): Refactor functions and update dependencies for improved codebase

[gianalarcon]

Overview
This PR makes several improvements to the codebase:

1. The greet function:

   • Moves string formatting inside the with_borrow closure
   • Eliminates an unnecessary clone operation
   • Reduces memory overhead

2. The set_greeting function:

   • Removes unnecessary braces in the closure
   • Maintains identical functionality with cleaner syntax

3. Updates the ic-cdk dependency from version "0.16.0" to "0.18.0"

Considerations
What impact will this change have on security, performance, users (e.g. breaking changes) or the team (e.g. maintenance costs)?

Both changes maintain identical functionality while making the code more concise and idiomatic.

[sesi200]

Thank you very much @gianalarcon! This PR could be merged as-is, but the ICP Ninja backend currently still struggles with the new CDK version. Because of that I will not quite yet merge this PR. ETA: 1-2 weeks