batches: revise query for changeset spec expiry

Created by: courier-new

Closes https://github.com/sourcegraph/sourcegraph/issues/20170, maybe.

Modifies the query for deleting changeset specs so that it will only flag a changeset spec that isn't attached to a changeset but is attached to an un-applied batch spec IF that changeset spec is also older than BatchSpecTTL.

I say this maybe closes that issue because I'm not sure if checking the age of the changeset spec is enough, or if I should actually be checking whether or not the associated batch spec is older than BatchSpecTTL. Would those more or less be equivalent checks in this case?

Since the batch spec deletion happens after the changeset deletion, I believe no action is needed to preserve the existing behavior for also deleting the batch specs associated with these changeset specs after BatchSpecTTL elapses.