Fix flaky spec in Projects::DisableLegacyInactiveProjectsService

What does this MR do and why?

This was reported in #446250 (closed)

Most likely the spec is flaky because we assume that the first N projects in inactive_public_projects are always migrated. But we don't have any deterministic ordering in the service. Also, I have updated the last_activity_at timestamp to 2 years ago as 1 year ago is just around the cut-off time used by the service it may be the reason behind this flaky behavior.

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How to set up and validate locally

Not sure how we can reproduce this.

Related to #456470 (closed) #446250 (closed)