Use actively maintained `backon` to replace `backoff`

[Xuanwo]

Hi, thanks for building async-openai. I'm starting this issue to discuss whether it's a good idea to replace backoff with the actively maintained backon. I noticed that this crate exposes backoff at the public API level. Therefore, I want to discuss it first before beginning any work.

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BackON has now reached version 1.0 and has only one dependency. It can be used in both WebAssembly and no_std environments.

I'm willing to help perform the migration if you think it's cool.

[64bit]

Hey, thank you for the offer to add support for backon, looks really nice project.

At the moment backoff gets the job done. However, it would be worth revisiting this in the future for WebAssembly and no_std environments. We can have this issue parked for now.

[mccormickt]

Just wanted to bump this issue now that there is an official advisory regarding backoff being unmaintained https://rustsec.org/advisories/RUSTSEC-2025-0012.

[spencerbart]

Second the bump. Cargo Deny throws errors due to the use of backoff.