Add missing availability to Borrow

[guoye-zhang]

Build failure on latest compiler snapshot (5-17) when testing apple/swift-async-algorithms#421:

/Volumes/Data/swift-async-algorithms/.build/checkouts/swift-collections/Sources/ContainersPreview/Types/Ref.swift:147:27: error: 'Ref' is only available in macOS 10.14.4 or newer
145 | 
146 | @available(*, deprecated, renamed: "Ref")
147 | public typealias Borrow = Ref
    |                  |        `- error: 'Ref' is only available in macOS 10.14.4 or newer
    |                  `- note: add '@available' attribute to enclosing type alias
148 | 
149 | @available(SwiftStdlib 5.0, *)

[sliemeobn]

also reported in #656

[lorentey]

Thank you! 👍