Update all copyright notices

[AlexBenny]

The Copyright Notice at the bottom of the LICENSE file is missing. It should look something like this: https://github.com/snowplow/snowplow-gtm-server-side-client/blob/main/LICENSE#L189

Also, we should update all the copyright headers (in source files):
* Copyright (c) 2014-2020 Snowplow Analytics Ltd. All rights reserved.
To:
* Copyright (c) 2014-2021 Snowplow Analytics Ltd. All rights reserved.

[paulboocock]

It should also be LICENSE rather than LICENSE-2.0.txt so that it is properly identified by Github.