changed backoff/strategies.go and backoff/strategies_test.go, only types and a function changed

**Description**

No features added. Just refactored backoff package to allow only types from time.Duration on time related variables.

**Tests**

Only modified a single test to use time.Duration instead of int.

**Metadata**

- Fixes #[[Link to Issue](https://github.com/ethereum-optimism/optimism/issues/6730#issuecomment-1678950667)]
Co-authored-by: Rcontre360 <rcontreraspimentel@gmail.com>