vpnmain.cgi: Reduce the number of offered ciphers

vpnmain.cgi: Reduce the number of offered ciphers

For new connections, we will now configure fewer ciphers by default. I
currently do not see any reason why we should support so many different
versions of AES-GCM and AES-128 by default.

The defaults should provide high security as well as decent
compatibility to solutions from other vendors.

I am currently not sure whether ChaCha20-Poly1305 should remain as
default as AES should usually outperform it by far. We can assume that
most hardware has support for AES-NI.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>