Added a NOTICE.txt

[seanstory]

Part of https://github.com/elastic/open-source/issues/423 part of https://github.com/elastic/enterprise-search-team/issues/4730

Adds a NOTICE.txt that describes the dependencies of the Data Extraction Service and the licenses under which we use them.

Checklists

Pre-Review Checklist

• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] this PR has a thorough description
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)