For anyone else who finds this wondering about a similar thing, I just wanted to add some extra context:
If you follow what the error message says here and try using Terraform 0.13 first then terraform init in that version should tell you what to do next, if any special actions are required. While I do recommend reading the upgrade guides, for this particular problem Terraform should lead you through the steps as you go, it just takes more than one step because the relevant upgrade tool is no longer included in Terraform v0.14.