Update to Xcode 16b2

[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/31
• [x] make sure SPIManifest SwiftVersion.latestRelease is pointing to the desired Swift version (for doc builds) (example PR)
• [x] update builder https://gitlab.com/finestructure/swiftpackageindex-builder/-/merge_requests/293
  • [x] SwiftVersion.xcodeVersion
  • SwiftVersion.dockerImage doesn't need changing, this is done via spi-images
• [x] update server (no change needed)
  • [x] SwiftVersion.xcodeVersion
• [x] remove old Xcode version from builders (not really needed anymore)

[finestructure]

Builds with Xcode 16b2 have started.