We will no longer support CentOS, Fedora, and Opensuse as installation targets. These are not actively maintained by anyone with commit rights and occasionally cause issues for users as they are not tested during the release process.
Rather than have subpar support for an untested operating system, we're removing support for them completely. Users of these operating systems should take a look migration to the docker-based installation method, which will always be tested and supported by the project.
Additionally, drop support for Debian 9 as it is now EOL.
This makes the installation a bit more secure by ensuring a user does not accidentally expose a way for unauthorized users to add new ssh keys to the system.
Additionally, this removes the extra HOSTNAME file to make the initial install process easier (that file was not modifiable by any dokku commands.
Closes#2247
Users should never be in a position where they need to manually edit files managed by Dokku, and should be pushed to using the `domains` plugin where necessary.
Closes#3064
[ci skip]
