fix: add required replace for replicas field in mongodb instance

Author: Marcel Jacek

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

@@ -250,6 +250,9 @@ func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, r
 			},
 			"replicas": schema.Int64Attribute{
 				Required: true,
+				PlanModifiers: []planmodifier.Int64{
+					int64planmodifier.RequiresReplace(),
+				},
 			},
 			"storage": schema.SingleNestedAttribute{
 				Required: true,