Use a map for root_block_device instead of list

[Yethal]

Is your request related to a new offering from AWS?

Is this functionality available in the AWS provider for Terraform? See CHANGELOG.md, too.

• No 🛑: please wait to file a request until the functionality is avaialble in the AWS provider
• Yes ✅: please list the AWS provider version which introduced this functionality

Is your request related to a problem? Please describe.

Instead of using a list of maps for root_block_device I'd like a map to be used. There can't be more than one root_block_device anyway and turning it into a single map will make it easier for terraform wrappers to merge in configuration
As suggested in #396, I'm opening an issue instead of keeping PR open.

Describe the solution you'd like.

Describe alternatives you've considered.

Additional context

[github-actions]

This issue has been automatically marked as stale because it has been open 30 days
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[Yethal]

unstale

[github-actions]

This issue has been automatically marked as stale because it has been open 30 days
with no activity. Remove stale label or comment or this issue will be closed in 10 days

[Yethal]

unstale