ovpnmain.cgi: Fixes Bug#13117 - adds legacy option to openssl commands for cert ...

ovpnmain.cgi: Fixes Bug#13117 - adds legacy option to openssl commands for cert & key extraction

- Any insecure connections made with openssl-3.x can have the cert and key extracted but
   if the insecure connection was made from prior to CU175 Testing then it used
   openssl-1.1.1 which causes an error under openssl-3.x due to the old version being able
   to accept older ciphers no longer accepted by openssl-3.x
- Adding the -legacy option to the openssl commands enables openssl-3.x to successfully
   open them and extract the cert and key
- Successfully tested on a vm system. Confirmed that the downloaded version under
   openssl-3.x worked exactly the same as the version downloaded under openssl-1.1.1

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>