update SPI manifest to build on 6.2 only (#196)

adds configuration to SwiftPackageIndex manifest to request builds
happen explicitly on Swift 6.2 vs whatever the current default it

Author: heckj

.spi.yml

@@ -2,3 +2,4 @@ version: 1
 builder:
   configs:
     - documentation_targets: [Containerization, ContainerizationEXT4, ContainerizationOS, ContainerizationOCI, ContainerizationNetlink, ContainerizationIO, ContainerizationExtras, ContainerizationArchive, SendableProperty]
+      swift_version: '6.2'