[ios] Update documentation and assertion about Xcode requirements

Building Chromium on iOS now requires access to iOS 15.0 SDK, so
update the documentation and the assert that enforces a correct
version of Xcode is used.

Bug: none
Change-Id: I5d77bf8682a19d7f3b01e7a2319646c57a7ebf3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3193013
Reviewed-by: Rohit Rao <[email protected]>
Commit-Queue: Sylvain Defresne <[email protected]>
Cr-Commit-Position: refs/heads/main@{#926262}
diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index 705e101..b735d704 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -12,8 +12,8 @@
 
 ## System requirements
 
-* A 64-bit Mac running 10.15.4 or later.
-* [Xcode](https://developer.apple.com/xcode) 12.2 or higher.
+* A 64-bit Mac running 11.3 or later.
+* [Xcode](https://developer.apple.com/xcode) 13.0 or higher.
 * The current version of the JDK (required for the Closure compiler).
 
 ## Install `depot_tools`