Speed up iOS/macOS CocoaPods packaging pipeline (#27279)
### Description
<!-- Describe your changes. -->
- Split out sysroot/arch framework builds into separate jobs.
Previously, they were all being built sequentially.
- Update build agent image to macOS-15 and update Xcode version
accordingly. This also seems to speed up the framework builds.
### 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. -->
Speed up the packaging pipeline build. The average duration of recent
pipeline runs in main is more than 4 hours. Here's a test run that took
1.5 hours:
https://aiinfra.visualstudio.com/Lotus/_build/results?buildId=1078804&view=results
