fix(ios): Set minimum XCode version in GitHub Action to 15

[GitToTheHub]

• The iOS 16.x Test failed, because the XCode version 14.x could not be found. A minimum XCode version would be 15.x.
• Made ios-version 15.x test equal with with the other tests. There is no need to run an older xcode version on an older macos version. Event the recent XCode version 26 does support iOS 15, so it's ok to use minimum XCode 15.

Platforms affected

Motivation and Context

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary