feat: expose main_version on /version endpoint

[nicoloboschi]

Sometimes with alpha releases or post release it might be useful to know which is the main version, without having the client to interpret the result.

This PR also caches the version value as static variable since the /version endpoint is called many times by the frontend

[github-actions]

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_{Last Modified at 25 Jul 24 07:51 UTC}

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[ogabrielluiz]

Thanks, Nicolo!

LGTM