README installation instructions install an old version of `mmctl`

[barrettj12]

The README says: https://github.com/mattermost/mmctl/blob/dacf19cdd5fb8d3dbc9a7daf954059ce794fa9fb/README.md?plain=1#L13-L17

However, running this command installs a very old version of mmctl (v6.4.2):

$ go install github.com/mattermost/mmctl/v6@master
go: downloading github.com/mattermost/mmctl/v6 v6.4.2-0.20230417141347-dacf19cdd5fb

At the very least, the README should give instructions for installing the latest version of mmctl. Even better would be giving instructions for installing a specified version, e.g. 7.5.2 or 7.8-latest.