MM-60606: Respect allowFromCache flag in Channelstore.GetMany (#28290)

Setting the flag to false never worked, and it was never
caught because there wasn't an instance when this method
was called with allowFromCache=false.

https://mattermost.atlassian.net/browse/MM-60606

```release-note
NONE
```

server/channels/store/localcachelayer/main_test.go

@@ -104,6 +104,7 @@ func getMockStore(t *testing.T) *mocks.Store {
 	mockChannelStore.On("Get", channelId, true).Return(&fakeChannel1, nil)
 	mockChannelStore.On("Get", channelId, false).Return(&fakeChannel1, nil)
 	mockChannelStore.On("GetMany", []string{channelId}, true).Return(model.ChannelList{&fakeChannel1}, nil)
+	mockChannelStore.On("GetMany", []string{channelId}, false).Return(model.ChannelList{&fakeChannel1}, nil)
 	mockChannelStore.On("GetMany", []string{fakeChannel2.Id}, true).Return(model.ChannelList{&fakeChannel2}, nil)
 	mockChannelStore.On("GetByNames", "team1", []string{fakeChannel1.Name}, true).Return([]*model.Channel{&fakeChannel1}, nil)
 	mockChannelStore.On("GetByNames", "team1", []string{fakeChannel2.Name}, true).Return([]*model.Channel{&fakeChannel2}, nil)