Add NOTICE.txt for 7.x integration branches (#5488) (#5549)

* Add NOTICE file

(cherry picked from commit 12eb3e6)

* Include recyclable memorystream in notice for 7.x

* Update with PR feedback

Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

Co-authored-by: Martijn Laarman <Mpdreamz@gmail.com>
Co-authored-by: Steve Gordon <sgordon@hotmail.co.uk>

NOTICE.txt

@@ -0,0 +1,59 @@
+Elasticsearch .NET clients
+Copyright 2012-2021 Elasticsearch B.V.
+
+==========
+Notice for: Utf8Json
+----------
+Based on the Utf8Json project by Yoshifumi Kawai,
+licensed under the MIT license. https://github.com/neuecc/Utf8Json/
+
+MIT License
+
+Copyright (c) 2017 Yoshifumi Kawai
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+==========
+Notice for: Microsoft.IO.RecyclableMemoryStream
+----------
+Based on the Microsoft.IO.RecyclableMemoryStream project by Microsoft,
+licensed under the MIT license. https://github.com/Microsoft/Microsoft.IO.RecyclableMemoryStream/
+
+MIT License
+
+Copyright (c) 2015-2016 Microsoft
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+