This ticket includes the creation of a "channel modify" subcommand in mmctl based on the Mattermost CLI command that can be found in cmd/mattermost/commands/channel.go.
This command can make use of the UpdateChannelPrivacy function of the model/client4.go file from the mattermost-server, and it should mimic the structure and the behavior of the Mattermost CLI, with the exception of the --username flag that the Mattermost CLI currently receives and will not necessary in mmctl.
As part of this ticket, the "channel make_private" subcomand should be removed from mmctl, as the creation of "channel modify" makes it obsolete.
mmctl command: `mmctl channel modify [channel] [flags]` e.g. `channel modify myteam:mychannel --private`
See `files changed` on the PR