Update Terraform terraform-aws-modules/iam/aws to v4.2.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
terraform-aws-modules/iam/aws (source)	module	minor	4.1.0 -> 4.2.0

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Release Notes

terraform-aws-modules/terraform-aws-iam

### [`v4.2.0`](https://togithub.com/terraform-aws-modules/terraform-aws-iam/blob/master/CHANGELOG.md#v420---2021-06-29) [Compare Source](https://togithub.com/terraform-aws-modules/terraform-aws-iam/compare/v4.1.0...v4.2.0) - feat: Support External ID with MFA in iam-assumable-role ([#​159](https://togithub.com/terraform-aws-modules/terraform-aws-iam/issues/159))

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