Update url and checksum in Package.swift with the 1.15.0 iOS package details (#16174)
### Description
<!-- Describe your changes. -->
Update Package.swift on the release branch with the url and checksum
from the release iOS pod.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Swift Package Manager refers to the file in the repo directly, and we
couldn't update the checksum until the release was done.