gnutls uses libidn2 internal symbols which were dropped

gnutls when compiled with an older to 2.0 libidn2 will use _idn2_punycode_decode which is not an exported symbol of the library and has been dropped in the later versions.

Relates to libidn/libidn2#74 (closed)

I'm proposing to no longer use this symbol in gnutls, and drop compatibility with older libidn2 versions.