Update ignore.d.server/postfix for backrefs

The backrefs in these two rules were broken when converting rules to support high-precision timestamps. Update the backref accordingly.

Example postfix log message from a debian trixie system:

2025-11-29T10:00:00.123456-08:00 mail postfix/smtpd[42]: warning: 0.2.0.192.zen.spamhaus.org: RBL lookup error: Host or domain name not found. Name service error for name=192.0.2.0.zen.spamhaus.org type=A: Host not found, try again

Fixes: 10755fa0 ("Support rsyslog high-precision timestamps (Closes: #1023755)")