Support `.macOS(.v10_15)`

[baksha97]

Given swift-syntax now supports .macOS(.v10_15), can we adjust to this min ver?

UserDefaultsObservation/Package.swift

Line 9 in 3ee37f5

platforms: [.macOS(.v14), .iOS(.v17), .macCatalyst(.v17), .tvOS(.v17), .watchOS(.v10)],