Replace COPYRIGHT file from the root with NOTICE file

The primary reason: make GitHub detect that we use Apache License 2.0
They do it via https://github.com/licensee/licensee Ruby library (gem).

Our COPYRIGHT file contains a part of the Apache License, which should
be added to a source file, not the license or copyright information itself,
which confuses the library.

Instead, the recommended way is to create a NOTICE file which references
license of the code and its bundled dependencies.

COPYRIGHT

@@ -1,20 +0,0 @@
-This software is licensed under the Apache 2.0 License:
-
-----------------------------------------------------------------------------
-Copyright 2021 Zenith Labs, Inc
-
-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.
-----------------------------------------------------------------------------
-
-The PostgreSQL submodule in vendor/postgres is licensed under the
-PostgreSQL license. See vendor/postgres/COPYRIGHT.

NOTICE

@@ -0,0 +1,5 @@
+Neon
+Copyright 2022 Neon Inc.
+
+The PostgreSQL submodule in vendor/postgres is licensed under the
+PostgreSQL license. See vendor/postgres/COPYRIGHT.