Use Swift 6 when available & upgrade to Xcode 16.2 #816

Job	Run time
build-eliza-swiftpm-example	1m 29s
build-library-ios	1m 51s
build-eliza-cocoapods-example	56s
build-library-watchos	1m 35s
build-plugin-and-generate	13s
build-library-tvos	1m 30s
build-library-macos	1m 51s
run-conformance-tests	2m 41s
run-swiftlint	14s
run-unit-tests	2m 13s
validate-license-headers	18s
	14m 51s

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