www/grafana8: Reflect upstream LICENSE change

Upstream has switched the license to AGPLv3 since Grafana 8:

https://github.com/grafana/grafana/commit/8db3eb90ae8c20e1a2bbbad300e0683def9bf8fb

Reported by: txt.file@txtfile.eu (by email)
Approved by: portmgr (blanket)
MFH: 2021Q3