Relicense to AGPL-3.0-or-later

Please check if the PR fulfills these requirements

[x] The PR has no duplicates (please search among the Pull Requests before creating one)
[x] The PR follows our contributing guidelines
[ ] Tests for the changes have been added (for bug fixes / features)

What kind of change does this PR introduce?

License change from Apache-2.0 to AGPL-3.0

What is the current behavior?

The Arduino Language Server is currently under the Apache-2.0 license, which is not compatible with the license of dependencies used by the project. This is highlighted by check-go-dependencies-task.

What is the new behavior?

As requested by @alranel, the Arduino Language Server has been re-licensed to AGPL-3.0. This is compatible with the license of dependencies used by this project. We did our best to assess the intellectual property of this codebase and make sure all contributors and copyright owners granted us permission to relicense their code. For any inquiry, report or claim please contact us at license@arduino.cc.

Other information:

@alranel can you please approve and merge this PR?

arduino / arduino-language-server