Use Ubuntu 20.04 for a newer Python version

See https://github.com/protocolbuffers/upb/issues/760
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index 7e2b911..d06e588 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,7 +1,7 @@
 ---
 tasks:
   ubuntu:
-    platform: ubuntu1804
+    platform: ubuntu2004
     shell_commands:
     - "sudo apt -y update && sudo apt -y install libreadline-dev cmake rsync"
     test_targets: