Raise availability to anyAppleOS 27.0

[inductor]

Motivation

swift-http-api-proposal is moving to swift-collections 1.7.1 and swift-async-algorithms 1.1.7. In those releases, the container protocols and AsyncStreaming (AsyncReader and friends) require 27.0 on Apple platforms, so HTTPAPIs is raising all of its availability annotations to anyAppleOS 27.0.

swift-http-api-proposal also compiles the NIOHTTPServer sources from this repository through a submodule, so declarations here that are still marked anyAppleOS 26.0 fail to build against the updated HTTPAPIs:

error: 'HTTPServerCapability' is only available in macOS 27.0 or newer

Modifications

• Raised every @available(anyAppleOS 26.0, *) to @available(anyAppleOS 27.0, *).
• Added @available(anyAppleOS 27.0, *) to the ByteBuffer extension in NIOHTTPServerResponseSender.swift, which uses RangeReplaceableContainer and had no availability annotation.
• Raised the three @available(anyAppleOS 26.2, *) annotations in HTTP3DatagramTests.swift to 27.0, since they call test helpers that now require 27.0.

Result

This package's availability matches HTTPAPIs, and swift-http-api-proposal can update its submodule to build against swift-collections 1.7.1.

Builds and tests pass on macOS with all traits enabled.

[aryan-25]

The CI failures should be resolved by #144.

[FranzBusch]

Test failures seems unrelated