docs: Switch to build.os for RTD

Uses `build.os` to build the documentation on rtd as `build.image` has
been deprecated.

[skip ci]

Author: st-bender

.readthedocs.yml

@@ -1,13 +1,14 @@
 version: 2
 
 build:
-    image: latest
+    os: "ubuntu-22.04"
+    tools:
+        python: "mambaforge-22.9"
 
 conda:
     environment: .rtd-environment.yml
 
 python:
-    version: "3.8"
     install:
       - method: pip
         path: .