Use Swift 6 when available & upgrade to Xcode 16.2 (#331) #825

Job	Run time
build-library-macos	1m 24s
build-eliza-cocoapods-example	44s
build-eliza-swiftpm-example	1m 22s
build-library-ios	2m 3s
build-library-tvos	1m 38s
build-library-watchos	1m 22s
run-conformance-tests	2m 24s
build-plugin-and-generate	1m 25s
run-swiftlint	9s
run-unit-tests	2m 9s
validate-license-headers	18s
	14m 58s

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