statefile: Ignore unknown check results on decode

[roni-frantchi]

Backport issue resolved in legacy Terraform 1.5.7.

Terraform 1.5 introduced a new check result kind, which causes a decode failure when read by Terraform 1.3 and 1.4. This means that a 1.5+ state file cannot be read by the terraform_remote_state data source in 1.3 or 1.4 series.

Aborting state decode at this point isn't strictly necessary, as we don't stand to lose important data by failing to round-trip these check results. Instead we can ignore those check results, allowing decode to elide them from the state.

This has no negative effect on the remote state data source, which only exposes root module outputs anyway.

Link to PR under 1.5.x MPL: hashicorp/terraform#33818