The simple answer is "the latest".
Cloud is updated irregularly and very frequently - I've seen the version change twice inside a week.
The version you are on is also of no practical use to you. You don't need to know it for anything.
That's one of the things about using Software Services instead of your own server installs - things like compatibilty and upgrades are handled by the provider and you shouldn't need to care.
If you absolutely must report back a specific version, then the easiest thing to do is write a short script that makes a REST call to your Cloud system to fetch the system information and extract the number from the response. But you'll need to run it frequently as the version will change a lot (Atlassian seem to have a cadence of 5-6 weeks between updates, but it can change a lot more often)
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