Default PowerLevelContent.Invite to 0 per matrix spec

The default power level for invite was wrong in matrix spec v1.1[1],
and has been fixed in v1.3, bringing us in line with synase[2].

[1] matrix-org/matrix-spec@136b5c9
[2] https://github.com/matrix-org/synapse/blob/v1.92.3/synapse/handlers/event_auth.py#L138

Signed-off-by: Nick Cao <nickcao@nichi.co>

eventcontent.go

@@ -429,9 +429,9 @@ func NewPowerLevelContentFromAuthEvents(authEvents AuthEventProvider, creatorUse
 }
 
 // Defaults sets the power levels to their default values.
-// See https://spec.matrix.org/v1.1/client-server-api/#mroompower_levels for defaults.
+// See https://spec.matrix.org/v1.8/client-server-api/#mroompower_levels for defaults.
 func (c *PowerLevelContent) Defaults() {
-	c.Invite = 50
+	c.Invite = 0
 	c.Ban = 50
 	c.Kick = 50
 	c.Redact = 50