Unknown certificate compression algorithm leads to an illegal_parameter alert

Advertizing a certificate compression not known to gnutls in CompressCertificateExtension leads to illegal_parameter instead of ignoring the extension and proceeding with no compression.

Code pointer: https://gitlab.com/gnutls/gnutls/-/blob/b69cbc76e46bbface6f92a0485a6c7ae646c6d6b/lib/ext/compress_certificate.c#L197

(Found using tlsfuzzer, though the corresponding code isn't mainlined yet.)

CC @ZoltanFridrich.