init: Add godoc comments for ForceCopy to mention that it's equivalent to -migrate-state in automation

[SarahFrench]

Related Issue

N/A

Description

I was investigating #355 (and #460) and realised that the ability to migrate state is already present in terraform-exec via the -force-copy flag.

There are potentially some behaviour changes to consider for very old Terraform versions; looks like init used to always attempt a state migration but then that feature was made optional behind -migrate-state. By then the -force-copy flag had already existed for several years. That makes sense; it allowed the older, automatic migration behaviour to work in automation. Once that automatic migration was removed it began representing consent to do the migration as well as suppressing prompts with yes.

In terms of terraform-exec I think we don't need to have any special consideration of the above and we can simply check that -force-copy is used in a compatible version of Terraform. It was added in TF v0.9.2 (see commit hashicorp/terraform@ddb9c51481e302d6cfde62f8475f5f8aaed90e35's associated tags)

In this PR

I've added some godoc comments to help users realise that -migrate-state isn't needed.
Also, I've updated some tests to assert that the flag can be changed to a non default value.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

N/A

[radeksimko]

Looks like the unit tests still need fixing - unsure why