[MM-60684] GetMaxPostSize: update threshold to V2, it was way overdue (#28662)

server/channels/app/platform/config.go

@@ -369,12 +369,7 @@ func (ps *PlatformService) IsFirstUserAccount() bool {
 }
 
 func (ps *PlatformService) MaxPostSize() int {
-	maxPostSize := ps.Store.Post().GetMaxPostSize()
-	if maxPostSize == 0 {
-		return model.PostMessageMaxRunesV1
-	}
-
-	return maxPostSize
+	return ps.Store.Post().GetMaxPostSize()
 }
 
 func (ps *PlatformService) isUpgradedFromTE() bool {