Catch Timeouts in UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker

What does this MR do?

subscription.refresh_seat_attributes! is causing ActiveRecord::QueryCanceled errors: https://sentry.gitlab.net/gitlab/gitlabcom/issues/2620964/events/41535946/

Catch ActiveRecord::QueryCanceled errors raised in the UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker. Track the errors, and continue updating remaining subscriptions.

Implements item 2 here: #329936 (comment 574893361)

Issue: #329936 (closed)

Does this MR meet the acceptance criteria?

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