Warning-as-error in Cosmos for deprecated function

When building with the nightly toochain, we get a warning that will be an error in build:

```
warning: use of deprecated method `std::sync::atomic::Atomic::::fetch_update`: renamed to `try_update` for consistency
--> sdk/cosmos/azure_data_cosmos_perf/src/runner.rs:757:10
|
757 | .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
| ^^^^^^^^^^^^
|
= note: `#[warn(deprecated)]` on by default
help: replace the use of the deprecated method
|
757 - .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
757 + .try_update(Ordering::Relaxed, Ordering::Relaxed, |current| {
|
```

Open sdk/cosmos/azure_data_cosmos_perf/src/runner.rs and inspect the deprecated atomic method at line 757. Replace the deprecated call as indicated by the nightly compiler warning, then build with the nightly toolchain and confirm the warning no longer appears.

Written by the indexing model from the issue text.