chore: update license

[edward-ly]

Now that the min-version is 31 thanks to #174, I think we can safely switch to the new license tag now since we don't support v30 or below anymore.

Although, I've also been wondering whether or not we should change the license to MIT or something else for this project. Technically, any apps based on the template app as it is now are supposed to be licensed under the AGPL as well, which might not be what every developer wants. What do you think?

[provokateurin]

Good point. It's a bit of a weird situation with such a template. Alternatively we'd need some kind of statement that the license is only an example and the actual license is MIT, but this gets weird and is probably hard to understand. I think we generally prefer AGPL-3.0-or-later, so having it as a default is nice.

@AndyScherzinger what do you think?

[AndyScherzinger]

One out of two are typically our suggestion for the license to pick for a php app:agpl3+ or mit.

Since it is currently agpl (vague and not precise but agpl) I would stick with agpl3+ SPDX identifier.

Else to be more flexible for the template if there are no external, non-employee contributions (so it is simpler to change the license) one could go for cc0 and add an explanation suggesting mit/agpl3+ for the actual app built upon the template.

Currently I don't think it is worth the confusion it might create. So I agree to @provokateurin assessments and would stick to agpl3+