CVE-2022-36009: Incorrect parsing of access level in gomatrixserverlib and dendrite

gomatrixserverlib is a Go library for matrix protocol federation. Dendrite is a Matrix homeserver written in Go, an alternative to Synapse. The power level parsing within gomatrixserverlib was failing to parse the "eventsdefault" key of the m.room.powerlevels event, defaulting the event default power level to zero in all cases. Power levels are the matrix terminology for user access level. In rooms where the "eventsdefault" power level had been changed, this could result in events either being incorrectly authorised or rejected by Dendrite servers. gomatrixserverlib contains a fix as of commit 723fd49 and Dendrite 0.9.3 has been updated accordingly. Matrix rooms where the "eventsdefault" power level has not been changed from the default of zero are not vulnerable. Users are advised to upgrade. There are no known workarounds for this issue.