fix(gossipsub): Changed unsubscribe_backoff to expect Duration

[sirandreww-starkware]

Description

Gossipsub config back-off takes a duration as a number of seconds instead of std::time::Duration, this PR fixes that.

Fixes #6120

Notes & open questions

Allows using sub second and more specific unsubscribe_backoff times

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[AgeManning]

This change looks good to me.

Although you have linked an issue, can you please add a description and motivation for this PR.

This PR is what is primarily linked to the squashed commit and is what users will lookup in the history. All our PRs should have an accurate description and motivation.

[AgeManning]

Also, we need to modify the CHANGELOG in gossipsub, which is partially why CI is failing.

Also this modifies the user-facing API and will need a version update to include it.

[AgeManning]

Looks good to me. Will need a version bump to handle the API change.

[elenaf9]

Will need a version bump to handle the API change.

v0.50.0 is not released yet, so no version bump is needed. Only the changelog entry needs to be fixed.

[elenaf9]

LGTM