bintray is being shut down 1st of May 2021, rabbitmq docs direct
users to packagecloud - so let's move.
For Victoria and below, includes I6a3ba599af3deedd450b4ffff6b86a0c04628340
(base: drop Linaro OBS key for CentOS and Ubuntu)
For Train, fixes also the order of rpm gpg key imports and
yum operations (otherwise CentOS 8 [with dnf] fails because repos
are enabled early and keys are missing).
And it also pins erlang version where required.
(cherry picked from commit