Update iOS build instructions.
Update iOS build instructions.

Updates information on managing newer versions of Xcode, and handling
older devices.

Change-Id: I6701c17268b013e3a57da931a73c49c25cb0bcfa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268630
Reviewed-by: Heather Miller <2e22000c5d22374561fe5fba576a31095b72dc2e@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

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