Update to Xcode 16b5

[finestructure]

• Package list does not need to be updated, because it uses image tags without patch versions and will auto-update to the latest available patch version
• The same is true for the validator
• [x] install new Xcode version on builders
• [x] Update spi-images https://gitlab.com/finestructure/spi-images/-/merge_requests/35
• [x] update builder https://gitlab.com/finestructure/swiftpackageindex-builder/-/merge_requests/306
  • [x] SwiftVersion.xcodeVersion
  • SwiftVersion.dockerImage doesn't need changing, this is done via spi-images

[finestructure]

Run #10 with Xcode 16.0b5 is under way.