Fix copyright notice in LICENSE

[felixbarny]

The year doesn't need to be a range of years. Just the start year is relevant.
We don't require contributors to transfer the copyright to us, so the copyright is for Elastic and contributors.

[apmmachine]

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• Start Time: 2021-10-11T09:39:16.147+0000

• Duration: 59 min 51 sec

• Commit: 4cd8ace

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Test	Results
Failed	0
Passed	2418
Skipped	19
Total	2437

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