[CoreExtension] Remove `NS_SWIFT_UNAVAILABLE` attribute that caused breaking change #3594

Job	Run time
spm-package-resolved	1m 15s
pod-lib-lint (ios, macos-14, Xcode_15.2)	10m 25s
catalyst	6m 53s
pod-lib-lint (ios, macos-14, Xcode_16)	7m 54s
pod-lib-lint (tvos, macos-14, Xcode_15.2)	8m 11s
pod-lib-lint (tvos, macos-14, Xcode_16)	5m 50s
pod-lib-lint (macos --skip-tests, macos-14, Xcode_15.2)	3m 59s
pod-lib-lint (macos --skip-tests, macos-14, Xcode_16)	2m 36s
pod-lib-lint (watchos, macos-14, Xcode_15.2)	2m 0s
pod-lib-lint (watchos, macos-14, Xcode_16)	1m 48s
spm (macos-13, Xcode_15.2, iOS)	6m 50s
spm (macos-14, Xcode_15.4, iOS)	8m 14s
spm (macos-15, Xcode_16, iOS)	2m 22s
spm (macos-15, Xcode_16, tvOS)	2m 0s
spm (macos-15, Xcode_16, macOS)	2m 12s
spm (macos-15, Xcode_16, watchOS)	2m 27s
spm (macos-15, Xcode_16, catalyst)	2m 2s
spm (macos-15, Xcode_16, visionOS)	2m 26s
core-cron-only	0s
	1h 19m 24s

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