Ensure version 3.7 works

[Gazler]

Since OTP 21 is now available from Erlang Solutions, it is installed by default. This requires RabbitMQ 3.7.7 and above.

As of version 3.7, the releases are stored in a different location, so the task has been modified to fetch the release from GitHub.

There is no way to customise the OTP version, so perhaps it is worth requiring a version of 3.7.7 or above and deleting the 3.6 version of the task?

It is also required to restart the rabbitmq server before running rabbitmqctl when using a cluster as the cookie does not match otherwise.