support storage class update in postgresflex (#348)

Author: DiogoFerrao

stackit/internal/services/postgresflex/instance/resource.go

@@ -224,6 +224,9 @@ func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, r
 				Attributes: map[string]schema.Attribute{
 					"class": schema.StringAttribute{
 						Required: true,
+						PlanModifiers: []planmodifier.String{
+							stringplanmodifier.RequiresReplace(),
+						},
 					},
 					"size": schema.Int64Attribute{
 						Required: true,