fix: add GNU AGPL license

This repository was always intended to be licensed under the AGPL,
however the default placeholder license information was never updated.
See https://tasks.opencraft.com/browse/BB-8356?focusedCommentId=316703&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-316703 (internal link).

Setting the license now should be fine, since all contributors appear to
have been active OpenCraft members.

Private-ref: https://tasks.opencraft.com/browse/BB-10680

Fixes: #43