DEV: Add stub flush_sw task to prevent rebuild errors (PR #10021)

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I’d add a “This is expected” to the warning so we don’t get new support requests because a new “warning” in on the output

To clarify, what I did was take the change from https://github.com/discourse/discourse/pull/8384/files#diff-52cfe83d7f9f1c952a7995dfb0f99341 and rip out the function body. That ‘rescue’ block isn’t ever going to execute.