Combine the sections about required software in the README

For stupidity reasons I had split up the "Supported Platforms" and the
"Required Software" without realising that these are actually entangled.
This change fixes that.

README.adoc

@@ -28,7 +28,7 @@ ansible-galaxy install -r requirements.yml
 ansible-playbook site.yml
 ----
 
-== Supported Platforms
+== Required Software
 
 The setup works on:
 
@@ -36,8 +36,6 @@ The setup works on:
 * Ubuntu
 * macOS
 
-== Required Software
-
 Please install the following:
 
 * `tofu` or `terraform`