Update RTD build image from ubuntu-20.04 to ubuntu-24.04

Agent-Logs-Url: https://github.com/egraphs-good/egglog-python/sessions/64f82f3b-6fe6-4d43-9e5e-958a34556ca1

Co-authored-by: saulshanabrook <1186124+saulshanabrook@users.noreply.github.com>

Author: Copilot

.readthedocs.yaml

@@ -7,7 +7,7 @@ version: 2
 
 # Set the version of Python and other tools you might need
 build:
-  os: ubuntu-20.04
+  os: ubuntu-24.04
   tools:
     python: "3.12"
     #     # You can also specify other tool versions: