stackplan: Use comparable version strings for plan version checking

Previously we were comparing version.String() with version.SemVer.String(),
which doesn't match because the latter excludes any prerelease version
suffix.

internal/stacks/stackplan/from_proto.go

@@ -32,7 +32,7 @@ func LoadFromProto(msgs []*anypb.Any) (*Plan, error) {
 		switch msg := msg.(type) {
 
 		case *tfstackdata1.PlanHeader:
-			wantVersion := version.String()
+			wantVersion := version.SemVer.String()
 			gotVersion := msg.TerraformVersion
 			if gotVersion != wantVersion {
 				return nil, fmt.Errorf("plan was created by Terraform %s, but this is Terraform %s", gotVersion, wantVersion)