IPv4 NNS regex allows for leading zeros

[roman-khimov]

Describe the bug
01.02.08.09 is considered to be valid IPv4 address by the checks added in #2262. This can be problematic if anyone is to use this IP (got from NNS contract) as an input for BSD or POSIX compliant inet_aton() and inet_addr() functions which will interpret chunks with leading zeros as octal values rather than decimal (and some other libraries can still interpret them as decimal).

Expected behavior
We better forbid leading zeros.

[roman-khimov]

But this notation is forbidden by IPAddress.TryParse() (somewhat surprisingly), so 01.02.08.09 is not actually accepted in the end by the current NNS contract. Still, 00.00.00.00 is and it's very easy to miss these nuances, so we better still fix the regex.