Update swift-collections to 1.7.1 and swift-async-algorithms to 1.1.7

[inductor]

Motivation

Follow-up to #191. As suggested there, we should move swift-async-algorithms past 1.1.5, which doesn't build against swift-collections 1.7.x (apple/swift-async-algorithms#455).

swift-async-algorithms 1.1.6 and later require swift-collections from: "1.7.0", so swift-collections has to move too. In swift-collections 1.7.x the container protocols require 27.0 on Apple platforms (SwiftStdlib 6.4). With swift-async-algorithms 1.1.6 that breaks the AsyncStreaming build on Apple platforms, and 1.1.7 (apple/swift-async-algorithms#459) fixes it by raising AsyncStreaming's availability to 27.0. Since HTTPAPIs is built on AsyncStreaming, its availability has to be raised to 27.0 as well.

Modifications

• Raised every @available(anyAppleOS 26.0, *) in Sources, Tests and Examples to @available(anyAppleOS 27.0, *).
• Updated swift-collections to exact: "1.7.1" and swift-async-algorithms to exact: "1.1.7". Both stay pinned with exact since their unstable traits can change in any release.
• Re-exported SpanPreview from HTTPAPIs alongside ContainersPreview, since swift-collections 1.7 moved span into SpanPreview.
• Updated the swift-http-server submodule to include Raise availability to anyAppleOS 27.0 swift-server/swift-http-server#143, which raises its availability to 27.0 in the same way.

Result

The package builds against swift-collections 1.7.1 and swift-async-algorithms 1.1.7. All APIs now require 27.0 on Apple platforms.

Test Plan

Builds and tests pass on macOS and on Linux.

[FranzBusch]

Thanks

[FranzBusch]

API breaks are expected. Marking this as minor since it bumps the platform annotations