testing/flare: fix license to AGPL-3.0-only
Per https://gitlab.com/Schmiddiii/flare/-/issues/47#note_1297810184, Flare is AGPL-3.0-only, not AGPL-3.0-or-later.
Per https://gitlab.com/Schmiddiii/flare/-/issues/47#note_1297810184, Flare is AGPL-3.0-only, not AGPL-3.0-or-later.