Github Actions の Xcode を 15.4 にあげる

[miosakuma]

• [UPDATE] GitHub Actions の Xcode のバージョンを 15.4 にあげる
  • 合わせて iOS の SDK を iphoneos17.5 にあげる

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This pull request includes updates to the GitHub Actions workflow and documentation to reflect changes in the Xcode version and iOS SDK used for builds.

GitHub Actions Workflow Update:

• Updated the Xcode version to 15.4 and the iOS SDK to iphoneos17.5 in the GitHub Actions workflow configuration file .github/workflows/build.yml. (XCODE and XCODE_SDK environment variables)

Documentation Update:

• Added a note in CHANGES.md to document the update of Xcode to version 15.4 and the iOS SDK to iphoneos17.5.

[miosakuma]

CI が通ったのでマージします。