Remove outdated information regarding Let's Encrypt ACME client behavior

[jnoordsij]

Per https://letsencrypt.org/2025/06/26/expiration-notification-service-has-ended Let's Encrypt no longer stores the email address provided via the ACME API for any kind of account-related updates:

We have deleted the email addresses provided to Let’s Encrypt via the ACME API that were stored in our CA database in association with issuance data. This doesn’t affect addresses signed up to mailing lists and other systems. They are managed in a separate ISRG system unassociated with issuance data.
Going forward, if an email address is provided to Let’s Encrypt via the ACME API, Let’s Encrypt will not store the address but will instead forward it to the general ISRG mailing list system unassociated with any account data. If the email address has not been seen before, that system may send an onboarding email with information about how to subscribe to various sources of updates.

Note that it may also be worthwhile exploring dropping this field entirely in a variety of examples here and elsewhere, however this is left out for the purpose of this PR.

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[erikgb]

Thanks @jnoordsij! 🚀 Feel free to submit a follow-up PR to clean out usage of the email field in the Let's Encrypt examples.

/lgtm
/approve

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