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.