[ios] Bump minimum version of Xcode required

With https://crrev.com/c/2722913, the build requires a version of
Xcode that includes macOS 11.0 SDK. It is not included with the
12.0 or 12.1 versions of Xcode, but is present in 12.2 or higher [1].

Update documentation accordingly (including the minimum version
of macOS required to be able to run Xcode according to Apple
documentation [1]).

[1]: https://developer.apple.com/documentation/xcode-release-notes/xcode-12_2-release-notes

Bug: none
Change-Id: I8605969bdc2633b6b46921507c2b8d5bea5fa847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2726712


Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858959}