rds/cluster/cluster_instance: engine becomes required

.changelog/31112.txt

@@ -0,0 +1,15 @@
+```release-note:breaking-change
+resource/aws_rds_cluster: The `engine` argument is now required and has no default
+```
+
+```release-note:note
+resource/aws_rds_cluster: Configurations not including the `engine` argument must be updated to include `engine` as it is now required. Previously, not including `engine` was equivalent to `engine = "aurora"` and created a MySQL-5.6-compatible cluster
+```
+
+```release-note:breaking-change
+resource/aws_rds_cluster_instance: The `engine` argument is now required and has no default
+```
+
+```release-note:note
+resource/aws_rds_cluster_instance: Configurations not including the `engine` argument must be updated to include `engine` as it is now required. Previously, not including `engine` was equivalent to `engine = "aurora"` and created a MySQL-5.6-compatible cluster instance
+```

internal/service/rds/cluster.go

@@ -174,9 +174,8 @@ func ResourceCluster() *schema.Resource {
  },
  "engine": {
  Type: schema.TypeString,
- Optional: true,
+ Required: true,
  ForceNew: true,
- Default: ClusterEngineAurora,
  ValidateFunc: validClusterEngine(),
  },
  "engine_mode": {

internal/service/rds/cluster_instance.go

@@ -99,9 +99,8 @@ func ResourceClusterInstance() *schema.Resource {
  },
  "engine": {
  Type: schema.TypeString,
- Optional: true,
+ Required: true,
  ForceNew: true,
- Default: ClusterEngineAurora,
  ValidateFunc: validClusterEngine(),
  },
  "engine_version": {