Replacement for block_duration_minutes with deprecation of spot blocks

#### Description

With the deprecation of spot blocks (spot instances with a defined duration) the `block_duration_minutes` parameter fails to work with AWS accounts that are not in grace period and will stop working December 31st, 2022 for everyone per [AWS Docs](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-requests.html#fixed-duration-spot-instances). The ask is to update to do the following:

- [ ] Update documentation to clarify the `block_duration_minutes` parameter cannot be used with new AWS accounts.
- [ ] Develop alternative flag to achieve same behavior, maybe something that removes spot instances from packer side after X amount of time?