Update README.md before archiving

[mosuem]

See dart-lang/http#1318

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
http2	None	2.3.1-wip	2.3.1-wip	2.3.1-wip	✔️

Changelog Entry ❗

Package	Changed Files
package:http2	README.md

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Coverage ✔️

File	Coverage

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API leaks ✔️

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Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Files
test/src/flowcontrol/mocks.mocks.dart