main/squid: upgrade to 5.6
http://www.squid-cache.org/Versions/v5/ChangeLog.html
Changes in squid-5.6 (06 Jun 2022):
- Bug 5208: Part 1: Restart kids killed by SIGKILL
- Fix SQUID-MIB smilint errors
- negotiate_kerberos_auth: Initialise default_keytab
- Improve handling of Gopher responses
- ... and some semi-automated code polish