Configure `authentication_events` to be `gitlab_main_clusterwide`

What does this MR do and why?

Related to #420392 (closed)

We want the AuthenticationEvent model to inherit from MainClusterwide::ApplicationRecord, as we are building cells and all tables that are closely associated with the User needs to be a clusterwide table as well.

Currently, this change has no effect, because MainClusterwide::ApplicationRecord only points to the main_clusterwide database for reading/writing only if that database is configured in database.yml, which we currently do not.

So, essentially, this change now acts as NO-OP.

Since there are no pipleline failures after this change, it is evident that there is no cross joins between namespaces table (the namespaces table is currently marked as gitlab_main_cell) and authentication_event table, so there are no cross-joins to be marked here for fixing later.

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Related to #420392 (closed)