Do not advertise Debian 9 support

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.

docs/faq.md

@@ -121,7 +121,7 @@ This is similar to the [EMnify/matrix-synapse-auto-deploy](https://github.com/EM
 
 - this one **can be executed more than once** without causing trouble
 
-- works on various distros: **CentOS** (7.0+), Debian-based distributions (**Debian** 9/Stretch+, **Ubuntu** 16.04+), **Archlinux**
+- works on various distros: **CentOS** (7.0+), Debian-based distributions (**Debian** 10/Buster+, **Ubuntu** 18.04+), **Archlinux**
 
 - this one installs everything in a single directory (`/matrix` by default) and **doesn't "contaminate" your server** with files all over the place
 

docs/prerequisites.md

@@ -4,7 +4,7 @@ To install Matrix services using this Ansible playbook, you need:
 
 - (Recommended) An **x86** server ([What kind of server specs do I need?](faq.md#what-kind-of-server-specs-do-i-need)) running one of these operating systems:
   - **CentOS** (7 only for now; [8 is not yet supported](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/300))
-  - **Debian** (9/Stretch or newer)
+  - **Debian** (10/Buster or newer)
   - **Ubuntu** (18.04 or newer, although [20.04 may be problematic](ansible.md#supported-ansible-versions))
   - **Archlinux**