added details for sebalis in contributor-key.xml

[sebalis]

I nominated myself for a credit in the next release (hope you agree) in #27037 (comment) – my Github handle was here since 5.56, now adding name and organisation.

[civibot]

Thank you for contributing to CiviCRM! ❤️ We will need to test and review the PR. 👷

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[colemanw]

Certainly! Thanks Sebastian for your contribution. Looking forward to the next one!