Upgrade readthedocs image to the latest one

22330288 · Virgile Andreani · Thomas Wiecki · 4692008f · 22330288
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -4,6 +4,6 @@ sphinx:
 conda:
  environment: doc/environment.yml
 build:
-  os: "ubuntu-20.04"
+  os: "ubuntu-lts-latest"
  tools:
-    python: "mambaforge-4.10"
+    python: "mambaforge-latest"