Issues and Pull Requests were not migrated to the new
organization/repository, so `matrix-org/synapse/pull` and
`matrix-org/synapse/issues` references were kept as-is.
`matrix-org/synapse-s3-storage-provider` references were also kept,
as that module still continues living under the `matrix-org` organization.
This patch mainly aims to change documentation-related things, not actual
usage in full yet. For polish that, another more comprehensive patch is coming later.
Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1282
It's mostly due to Docker CE dropping its repositories (and support) for
Debian 9.
If one installs Docker manually (likely a package named `docker.io`), it
will likely still work.
In any case, Debian 9 is old and end-of-life now, so advertising support
for it is not productive.
Slavi Pantaleev