redmine: set session_store to active_record_store
Change the session store to fix the
"ActionDispatch::Cookies::CookieOverflow" errors we are getting.
IMPORTANT: after deploying this change, the following commands must be
executed once manually to create the session table:
# docker compose exec redmine bash
$ rails generate active_record:session_migration
$ rake db:migrate
Related: OS#5476
Change-Id: Iacec800b9179cfac894f41e9eb1a5cca7e56ff1c
