Write down and announce Squiggle deprecation policy for Squiggle Hub

[berekuk]

Depends on #3482.

Something like...

• we will stop supporting v0.8.5 on Squiggle Hub in X months
• you have N models that use that version
• you can upgrade them here (custom bulk upgrade UI? or just separate links)
• in the future, upgrades will usually happen automatically, but if your model is private we won't be able to upgrade it for you
• if you don't upgrade then the behavior implemented in Gracefully handle unsupported Squiggle versions in Hub #3482 will happen
• (optionally: you can set this setting in your user profile to auto-upgrade in the future? or disable these emails?)