Change shards.yml license to SPDX identifier

[Fijxu]

The current license we have on shard.yml is:

invidious/shard.yml

Line 41 in 325e013

license: AGPLv3

However, the shards spec says:

license
An SPDX license expression or an URL to a license file (string, recommended)

So we should use something like AGPL-3.0-or-later or AGPL-3.0-only according to the list of licenses listed here. Is Invidious later or only?

[SamantazFox]

Is Invidious later or only?

Invidious is AGPL-3.0-only