Switch back to using main branch of RMG-database

The PR there was just merged in 
ReactionMechanismGenerator/RMG-database#667

Author: rwest

.github/workflows/CI.yml

@@ -47,7 +47,7 @@ concurrency:
 env:
   # if running on RMG-Py but requiring changes on an un-merged branch of RMG-database, replace
   # main with the name of the branch
-  RMG_DATABASE_BRANCH: lithium_rebase
+  RMG_DATABASE_BRANCH: main
 
 
 jobs: