builtin: Disable Terraform provider version checks

The builtin Terraform provider's remote state data source uses a
configured backend to fetch a given state, in order to allow access to
its root module outputs. Until this commit, this was only possible
with remote states which are from the current Terraform version or
older, forcing multi-state users to carefully orchestrate Terraform
upgrades.

Building on previous commits in this branch, we now disable this version
check, and allow any Terraform state file that the current Terraform
version can parse. Since we are only ever accessing root module outputs,
this is very likely to be safe for the foreseeable future.

builtin/providers/terraform/data_source_state.go

@@ -107,7 +107,7 @@ func dataSourceRemoteStateRead(d cty.Value) (cty.Value, tfdiags.Diagnostics) {
 		workspaceName = workspaceVal.AsString()
 	}
 
-	state, err := b.StateMgr(workspaceName)
+	state, err := b.StateMgrWithoutCheckVersion(workspaceName)
 	if err != nil {
 		diags = diags.Append(tfdiags.AttributeValue(
 			tfdiags.Error,
@@ -118,7 +118,7 @@ func dataSourceRemoteStateRead(d cty.Value) (cty.Value, tfdiags.Diagnostics) {
 		return cty.NilVal, diags
 	}
 
-	if err := state.RefreshState(); err != nil {
+	if err := state.RefreshStateWithoutCheckVersion(); err != nil {
 		diags = diags.Append(err)
 		return cty.NilVal, diags
 	}

builtin/providers/terraform/data_source_state_test.go

@@ -166,6 +166,26 @@ func TestState_basic(t *testing.T) {
 			}),
 			false,
 		},
+		"future version": {
+			cty.ObjectVal(map[string]cty.Value{
+				"backend": cty.StringVal("local"),
+				"config": cty.ObjectVal(map[string]cty.Value{
+					"path": cty.StringVal("./testdata/future.tfstate"),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"backend": cty.StringVal("local"),
+				"config": cty.ObjectVal(map[string]cty.Value{
+					"path": cty.StringVal("./testdata/future.tfstate"),
+				}),
+				"outputs": cty.ObjectVal(map[string]cty.Value{
+					"foo": cty.StringVal("bar"),
+				}),
+				"defaults":  cty.NullVal(cty.DynamicPseudoType),
+				"workspace": cty.NullVal(cty.String),
+			}),
+			false,
+		},
 		"missing": {
 			cty.ObjectVal(map[string]cty.Value{
 				"backend": cty.StringVal("local"),

builtin/providers/terraform/testdata/future.tfstate

@@ -0,0 +1,12 @@
+{
+    "version": 4,
+    "terraform_version": "999.0.0",
+    "serial": 0,
+    "lineage": "",
+    "outputs": {
+        "foo": {
+            "value": "bar",
+            "type": "string"
+        }
+    }
+}