[Maintenance] Replace unmaintained `backoff` crate with `backon` (RUSTSEC-2025-0012)

[pranc1ngpegasus]

What are you really trying to do?

Keep the Rust SDK's dependency tree free of unmaintained crates reported by cargo audit / RustSec.

Describe the issue

The SDK currently depends on the backoff crate for exponential retry delays. RUSTSEC-2025-0012 (RustSec advisory) reports that backoff is unmaintained and recommends migrating to backon for exponential backoff/retry.

backoff is used in:

• temporalio-client — gRPC call retry (TonicErrorHandler in crates/client/src/retry.rs)
• temporalio-sdk-core — poller autoscaling error backoff (crates/sdk-core/src/pollers/poll_buffer.rs)

Proposed change

Replace backoff with backon in both crates, using backon::ExponentialBuilder / ExponentialBackoff (Iterator::next() for delays). Each crate should depend on backon directly so sdk-core does not need to route retry utilities through client.

Additional context

• OSV: https://osv.dev/vulnerability/RUSTSEC-2025-0012
• RustSec: https://rustsec.org/advisories/RUSTSEC-2025-0012
• Suggested replacement: https://crates.io/crates/backon