It doesn't matter - the version changes frequently, and is essentially always going to be "latest". There are a handful of add-ons that Atlassian support on Cloud, but you don't need to think about versions on them - Atlassian handle it for you - on an upgrade, they either make sure a valid release is there or (rarely) deprecate the add-on, so you couldn't use it anyway.
Any extensions you can add to Cloud are either things that run externally or Connect add-ons. In both cases, they connect via the REST API which is quite stable (new versions add functions, but very very rarely change or delete functions), and you can simply look in the marketplace to evaluate if they're compatible with Cloud.
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