ci(ios): update workflow w/ iOS 15

[erisu]

Platforms affected

iOS

Motivation, Context & Description

Previously all iOS test appeared to run only on iOS 14.4.

This PR:

• Update GitHub Actions workflow to ensure that each defined iOS version was being tested correctly.
• Included iOS 15.x to the workflow.

Testing

• CI

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