Use Swift 6 when available & upgrade to Xcode 16.2 #822

Job	Run time
build-eliza-cocoapods-example	57s
build-library-macos	1m 29s
build-eliza-swiftpm-example	1m 34s
build-library-ios	1m 28s
build-library-tvos	1m 17s
build-library-watchos	1m 40s
run-conformance-tests	2m 19s
build-plugin-and-generate	1m 10s
run-swiftlint	10s
run-unit-tests	2m 3s
validate-license-headers	17s
	14m 24s

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