(maint) Update module_release_prep.yml to run on latest

Prior to this commit the release prep job is running Ubuntu 20.04 which will be removed from GH Actions on the 1st April. Updating to latest to run in line with all other jobs.

Author: pmcmaw

.github/workflows/module_release_prep.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   release_prep:
     name: "Release prep"
-    runs-on: "ubuntu-20.04"
+    runs-on: "ubuntu-latest"
 
     env:
       BUNDLE_WITHOUT: development:system_tests