Update/clarify data retention policy on billing page
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First of all, thank you very much for a privacy-friendly analytics service that is free for non-commercial use. I just started trying it out on a community site, so far I like it and will very likely use/recommend the commercial plan in the future.
The issue is that the billing page currently says
> What happens if I go over the amount of pageviews for my plan?
>
> ...
>
> Free plan: two years at maximum pageviews.
> Starter and business plans: four years at maximum pageviews.
I know this is not strictly about length of data retention, but it does give the impression that data is stored for the specified number of years if monthly pageviews stay within limit. I only found out that the data retention period is 6 months for Free and 12 months for Starter when I read [Updates](https://code.goatcounter.com/updates), which presumably most people would miss. I think it's a completely fair policy, but it could be made more clear on the billing/plans page.