Add 5.1 to the version list

Part of the 5.1 release tasks the [week before the release](https://moodledev.io/general/development/process/release#1-week-prior)

Author: Mihail Geshoski

.github/workflows/publish-apidocs.yml

@@ -1,8 +1,8 @@
 name: Generate API docs and publish to Github Pages
 
 env:
-  BRANCHLIST: "MOODLE_401_STABLE MOODLE_404_STABLE MOODLE_405_STABLE MOODLE_500_STABLE main"
-  VERSIONLIST: "4.1 4.4 4.5 5.0 main"
+  BRANCHLIST: "MOODLE_401_STABLE MOODLE_404_STABLE MOODLE_405_STABLE MOODLE_500_STABLE MOODLE_501_STABLE main"
+  VERSIONLIST: "4.1 4.4 4.5 5.0 5.1 main"
 
 on:
   schedule: