backup.pl: Fixes Bug#13137 - Existing n2n client connection created with openssl...

backup.pl: Fixes Bug#13137 - Existing n2n client connection created with openssl-1.1.1x fails to start with openssl-3.x

- This code adds the "providers legacy default" line into OpenVPN N2N Client config files
   when restoring them in case it is missing from a backup earlier than CU175.
   Only adds the line if it is not already present.
- Tested out on my vm testbed system

Fixes: Bug#13137
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>