Update supported distros list

I've just tested Rocky Linux v9 and it seems to work.

I suppose the Docker situation
(#300)
on RHEL v8 has improved, so it probably works too.

I see no reason AlmaLinux and other RHEL derivatives wouldn't work,
but I have neither tested them, nor have confirmation from others about
it.

It's mostly a matter of us being able to install:
- Docker, via https://github.com/geerlingguy/ansible-role-docker which
  seems to support various distros
- a few other packages (systemd-timesyncd, etc).

The list of supported distros has been reordered alphabetically.

I've heard reports of SUSE Linux working well too, so it may also be added
if confirmed again.

Closes #300

docs/prerequisites.md

@@ -2,11 +2,11 @@
 
 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** (10/Buster or newer)
-  - **Ubuntu** (18.04 or newer, although [20.04 may be problematic](ansible.md#supported-ansible-versions))
+- (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 that make use of [systemd](https://systemd.io/):
   - **Archlinux**
+  - **CentOS**, **Rocky Linux**, **AlmaLinux**, or possibly other RHEL alternatives (although your mileage may vary)
+  - **Debian** (10/Buster or newer)
+  - **Ubuntu** (18.04 or newer, although [20.04 may be problematic](ansible.md#supported-ansible-versions) if you run the Ansible playbook on it)
 
 Generally, newer is better. We only strive to support released stable versions of distributions, not betas or pre-releases. This playbook can take over your whole server or co-exist with other services that you have there.