RFR: 7904024: jcstress: Update concurrency mailing list in README.md

[Aleksey Shipilev]

On Sun, 8 Jun 2025 12:47:03 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

> In addition to updating the concurrency mailing list [^1], I also updated the address for this project's list [^2] [^3].
> 
> [^1]: https://mail.openjdk.org/pipermail/jcstress-dev/2025-June/001294.html
> [^2]: https://mail.openjdk.org/pipermail/announce/2022-May/000318.html
> [^3]: https://mail.openjdk.org/pipermail/announce/2022-June/000321.html

Marked as reviewed by shade (Committer).

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PR Review: https://git.openjdk.org/jcstress/pull/163#pullrequestreview-2912221427