BUILD: ssl: ASN1_STRING_length() is deprecated since OpenSSL 4.1

BUILD: ssl: ASN1_STRING_length() is deprecated since OpenSSL 4.1

OpenSSL 4.1.0-alpha1 deprecates ASN1_STRING_length() in favor of
ASN1_STRING_get_length(), which only differs by returning a size_t
instead of an int. This breaks the build when warnings are fatal:

  src/ssl_sock.c:4655:9: error: 'ASN1_STRING_length' is deprecated: Since OpenSSL 4.1; use ASN1_STRING_get_length() [-Werror=deprecated-declarations]

Let's simply remap the old name on the new function when built with
OpenSSL 4.1 and above, all the call places being compatible with both
return types.

This could be backported to all versions where we want to support
OpenSSL 4.1.