Update iOS minimum version documentation to 12.0 - #10001

Visit the preview URL for this PR (updated for commit 6b0c7bc):

https://flutter-docs-prod--pr10001-ios-12-yypem18h.web.app

If this is going to be landing on the stable channel soon, I'd actually consider merging this sooner rather than waiting for that. That way users can have up to date expectations and potentially avoid the issue.

I think I'd rather wait and submit this as simultaneously as possible with the hotfix since the supported versions page reflects stable:

As of the current stable release, support for deploying Flutter apps is shown in the following table:

Until that merges, users would have to manually edit some files to change their minimum from the default (11) to iOS 12.

Okay this is ready, merging on @parlough's LGTM. Thanks!

Thank you! 🚀