Migrate from deprecated Polly.Extensions.Http to Microsoft.Extensions.Http.Resilience

## Summary

`Elsa.Api.Client` depends on `Polly.Extensions.Http` version `3.0.0`, which NuGet's own package metadata marks as deprecated ("Legacy") with an explicit suggested alternative: `Microsoft.Extensions.Http.Resilience`.

Unlike some other dependencies, this isn't a security or supply-chain concern — `Polly.Extensions.Http` is still normally listed on NuGet and the Polly project (`App-vNext/Polly`) is actively maintained. It's simply the old ASP.NET Core HttpClient integration package that the Polly team retired in favor of the newer resilience pipeline API, and no version beyond 3.0.0 was ever published.

## Impact

Every consumer of `Elsa.Api.Client` inherits this as a transitive dependency, which shows up as a permanent "deprecated package" warning in `dotnet list package --deprecated` for any project referencing it — with no way to resolve it downstream, since there's no newer version to pin to.

I checked the current release (3.7.1) and the newest tag in the repo (3.8.0-preview1), and both still reference `Polly.Extensions.Http` 3.0.0 unchanged.

## Ask

Would it be possible to migrate `Elsa.Api.Client`'s HTTP resilience setup from `Polly.Extensions.Http` to `Microsoft.Extensions.Http.Resilience`? This would remove the deprecated-package warning for all downstream consumers. Happy to help test if useful.