feat: use modern data-ciphers naming while maintaining 2.4 compatibility (#1363)

## Summary

- Add `data-ciphers` directive alongside `ncp-ciphers` for
future-proofing
- Server config now emits both `data-ciphers` and `ncp-ciphers`
- Client config adds `ignore-unknown-option data-ciphers`,
`data-ciphers`, and `ncp-ciphers` for full backward compatibility with
OpenVPN 2.4 clients

## Context

The `ncp-ciphers` option is a legacy alias of `data-ciphers` that is
still accepted but deprecated in OpenVPN 2.5+. This change aligns with
modern naming conventions while maintaining compatibility with older 2.4
clients.

openvpn-install.sh

@@ -1240,6 +1240,8 @@ cert $SERVER_NAME.crt
 key $SERVER_NAME.key
 auth $HMAC_ALG
 cipher $CIPHER
+ignore-unknown-option data-ciphers
+data-ciphers $CIPHER
 ncp-ciphers $CIPHER
 tls-server
 tls-version-min 1.2
@@ -1389,6 +1391,9 @@ verify-x509-name $SERVER_NAME name
 auth $HMAC_ALG
 auth-nocache
 cipher $CIPHER
+ignore-unknown-option data-ciphers
+data-ciphers $CIPHER
+ncp-ciphers $CIPHER
 tls-client
 tls-version-min 1.2
 tls-cipher $CC_CIPHER