Log Message:
rabbitmq: Update to 3.7.16.

Changes since 3.7.7 are too long to list in full, but are primarily bug
fixes and usability improvements, along with the main reason for this
upgrade - it now builds against the current versions of erlang and elixir
delivered in pkgsrc.

Full changes are available from the following links:

  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.16
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.15
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.14
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.13
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.12
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.11
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.10
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.9
  https://github.com/rabbitmq/rabbitmq-server/releases/tag/v3.7.8