Add IBAN to sponsor footer

[oalders]

Logo Link: https://letsencrypt.org/images/sponsors/small/iban-logo.png

Description: iban.com provides enterprise-grade Software as a Service
(SaaS) solutions for payment validation, Verification of Payee (VoP),
and data retrieval, including comprehensive services for validating
International Bank Account Numbers (IBAN), Bank Identifier Codes (BIC),
and other global banking details. They offer an easy-to-integrate API to
help businesses reduce failed payments and associated bank charges.