NOTICE: add the original gpbackup NOTICE

[tuhaihe]

In our 2.1.0-rc2 review round, Jean Baptiste Onofre suggested we can include the original gpbackup NOTICE file content to the NOTICE. I also check the ASF docs, and find that we should do as the suggestion.

You can see more details here:

• https://lists.apache.org/thread/s3r9zmkt90cjnx8921mvmojw2zdfgh8k
• https://infra.apache.org/licensing-howto.html#mod-notice

closes: #ISSUE_Number

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[leborchuk]

Yes, it is instructed to add NOTICE info about copyrights if it was removed. And we could have a single NOTICE file, not to copy multiple files

Copyright notifications which have been [relocated](https://www.apache.org/legal/src-headers.html#headers), rather than removed, from source files must be preserved in NOTICE. However, elements such as the copyright notifications embedded within BSD and MIT licenses [do not need to be duplicated](https://issues.apache.org/jira/browse/LEGAL-59) in NOTICE. You can leave those notices in their original locations.