Update .readthedocs.yaml: use ubuntu-lts-latest instead of ubuntu-20.04 (#241)

* Initial plan

* Replace ubuntu-20.04 with ubuntu-lts-latest in .readthedocs.yaml

Agent-Logs-Url: https://github.com/glotzerlab/signac-docs/sessions/0d261ea3-50eb-4857-a608-44cad900322d

Co-authored-by: bdice <3943761+bdice@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bdice <3943761+bdice@users.noreply.github.com>

Author: Copilot

.readthedocs.yaml

@@ -1,6 +1,6 @@
 version: 2
 build:
-    os: ubuntu-20.04
+    os: ubuntu-lts-latest
     tools:
         python: "3"
 python: