Migrate readthedocs build to ubuntu-24.04

Readthedocs hasn't given any indication that ubuntu-20.04 will be deprecated, but it's approaching end of life (without paid support).

Author: Ortham

.readthedocs.yaml

@@ -6,7 +6,7 @@ sphinx:
 formats: all
 
 build:
-  os: ubuntu-20.04
+  os: ubuntu-24.04
   tools:
     python: '3.10'