Migrate to Elastic License v2

I seem to recall having found an issue on this repo a while ago where the same issue was raised, but where it was mentioned that it would likely not happen yet - it might have been on the Slack. I'm not too sure, and I couldn't find it when searching.

Anyway, the gist of my request is that the current license in this repo (the old ELv1 license) was added 6 years ago in #1, and a lot of things have happened since then. A year after, in 2021, we got the [new Elastic License v2](https://www.elastic.co/blog/elastic-license-v2) - and back in 2024, [AGPL was added as an option to Elasticsearch](https://www.elastic.co/blog/elasticsearch-is-open-source-again).

Isn't it about time that we update the license here to ELv2? After all, elastic-package seems quite well-used and recommended:
- https://www.elastic.co/blog/getting-started-with-elastic-package
- https://www.elastic.co/docs/extend/integrations/build-create-package