feat: show annual estimate and annual discount on pricing page

[xrdt]

Changes

Adds back the annual pricing toggle and annual plan discount when users estimate >$1,667k monthly / >$20k annual spend in the pricing calculator.

https://github.com/PostHog/posthog.com/assets/21014901/3e0cdd6a-90cc-45f8-9cd0-49d9c188e6b0

Checklist

• [ ] Words are spelled using American English
• [ ] Titles are in sentence case
• [ ] Feature names are in sentence case too
• [ ] Use relative URLs for internal links
• [ ] If I moved a page, I added a redirect in vercel.json
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• [ ] I've added this to the relevant "Tutorials and guides" docs page (if applicable)

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