Stop pinning `idna` and `url` crates (#3972)

## Motivation and Context
Now that we're on MSRV 1.8.1 we can addresses [these
TODOs](https://github.com/smithy-lang/smithy-rs/blob/main/aws/sdk/build.gradle.kts#L506-L507).

## Testing
Manually ran
```
./gradlew aws:sdk:cargoUpdateAllLockfiles 
```
and bulit `aws-sdk-s3` (can be any SDK crate) with updated `idna` and
`url` crates:
```
➜  s3 git:(ysaito/remove-pinning-idna-and-url) ✗ cargo check --all-features
...
    Checking idna v1.0.3
...
    Checking url v2.5.4
...
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 15.56s
```

I didn't include updated lockfiles in this PR as they will be updated
automatically next week by the bot's weekly PR to update lockfiles.

----

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