Migrate pallet-grandpa to attribute macros (#8724)

* frame/grandpa: migrate Config

* frame/grandpa: migrate decl_module

* frame/grandpa: migrate decl_event

* frame/grandpa: migrate decl_error

* frame/grandpa: migrate decl_storage

* frame/grandpa: make report_equivocation_unsigned pub(super)

* frame/grandpa: remove unused imports

* frame/grandpa: replace deprecated Module with Pallet

* frame/grandpa: add RawEvent for compatibility

* frame/grandpa: create migration to new storage prefix

* frame/grandpa: bump version to 4.0.0

* frame/grandpa: address review comments

* Try using version 3.1 instead

* frame/grandpa: tweak log text to say cancelled

substrate/frame/grandpa/src/migrations.rs

@@ -0,0 +1,19 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// Version 3.1.
+pub mod v3_1;