Include "Elastic-2.0" to the list of identifiers

[Jeremy Lempereur]

Elastic-2.0 has become a valid spdx identifier on February 3, 2021.

I would like to submit a PR that adds it to the identifiers with the 0x0 flag.

Edit:

I just updated spdx locally, using 3.18, and I can also add a changelog entry and update the readme, but I would totally understand if you would rather do it yourself.

Let me know if you would like a PR :)