RFR: 8327493: Update minimum Xcode version in docs

Tue Mar 26 09:24:29 UTC 2024

The file building.{md,html} indicates the required minimum version of Xcode to use. When the required minimum version of Clang was updated to 13 ([JDK-8325878](https://bugs.openjdk.org/browse/JDK-8325878)), the minimum Xcode version was not updated. It should now be Xcode 13.0, rather than 8.

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Commit messages:
 - 8327493: Update minimum Xcode version in docs

Changes: https://git.openjdk.org/jdk/pull/18483/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18483&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327493
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18483.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18483/head:pull/18483

PR: https://git.openjdk.org/jdk/pull/18483