RFR: Update mailman address

[Iris Clark]

On Fri, 1 Aug 2025 16:50:33 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

> Recently, a person sent an email to `mailman-at-openjdk.org` to change the email address as mentioned in the Guide, yet the email was rejected. The rejection email contained another address `mailman-owner-at-openjdk.org` that worked well. Therefore, I suggest updating the mail-man address in the Guide.
> 
> I also replaced “discussions _around_” with “discussions _about_”. This is how mailing lists are described on the page that lists all available OpenJDK mailing lists.

Updated e-mail address confirmed.

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Marked as reviewed by iris (Reviewer).

PR Review: https://git.openjdk.org/guide/pull/154#pullrequestreview-3085281534