Release 💎 v3.9.3

[parkr]

This is a 🚀 release.

Summary

Add release post, history, and update version.rb for 💎 release v3.9.3.

After this is reviewed & approved, I'll forward-port it to master.

Context

Truly closes #9268.

[ashmaroli]

Hello @parkr,
I would like to the following changes made:

• Let's not highlight the bit about non-half hour offsets --- the module is only used when built using Windows platform; its not a fix per se because it was not a feature prior to this. Let it be a sort of an Easter egg for those few who stumble upon it.
• I think the release post should also state that this is partly a security release as well because the loosened version-constraints on i18n gem was made to allow updating activesupport to a more recent version.

Other than the above, please consider the pull request approved and proceed to ship the release at your discretion.

[parkr]

Removed the bit about non-half-hour offsets. I don't think I need to mention that it's a security release since we didn't patch anything of our own, and none of the dependencies we use had a security vulnerability. This just fixes the mismatch so that folks can upgrade, but not all users of Jekyll will need to upgrade (so I don't want to scare them).