Once Flutter stable no longer supports iOS 8, we should update all the 1P plugins to match. This will eliminate the issue of deprecation warnings from the plugins in the now-common case where people build with projects that use iOS 9 as the minimum.

Currently all of our plugins support iOS 8 so this will affect every plugin that supports iOS, although we'll only need to update the deprecated plugins if at some point we have CI issues with the iOS 8 support.

When we do this, we should make sure the READMEs prominently list the minimum iOS version (see also #34999)