chore: Update the NOTICE file (vectordotdev#17430)

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Author: bruceg

NOTICE

@@ -1,4 +1,7 @@
+Unless explicitly stated otherwise all files in this repository are licensed under the Mozilla
+Public License, version 2.0 (MPL-2.0).
 
+This product includes software developed at Datadog (https://www.datadoghq.com/)
 
    Copyright (c) 2020 Vector Authors <[email protected]>