Update forum links that concierge looks for

[OmarShehata]

Concierge still looks for groups.google.com as forum links when an issue is closed, like here: CesiumGS/cesium#8175 (comment).

We should change that to community.cesium.com. We could even make concierge automatically post that message instead of asking us to post it, since there is a Cesium Concierge bot on the forum: https://community.cesium.com/u/cesium_concierge/summary.