Return suggested version along with recommendation

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR augments Gitlab::Ci::RunnerUpgradeCheck#check_runner_upgrade_status so that it returns a tuple of { <status>: <version> } instead of simply <status>. This makes it easier to reason about specs, and also will allow surfacing this information in the UI later on.

Closes #366177 (closed)

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