ios: Update docs to reflect non-support for iOS 8.x.
ios: Update docs to reflect non-support for iOS 8.x.

We actually dropped support for iOS 8 in the Xcode config a couple of
months ago, with commit 06939df1d; that should mean that new builds
don't appear in the App Store for older versions.  We neglected to
update our own docs, though; fix that.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile