Fix IPv6 routing by replacing prefixes with /56

[Noki]

With the switch to bird we stopped redistributing small prefixes to minimalize routing table size. The following locations require a replacement of the prefix with an /56 prefix so IPv6 works again:

• dragonkiez-adlerhalle.yml:ipv6_prefix: "2001:bf7:830:b3c0::/58" (dragonkiez: /56 subnets for all cores #1074) - flashed
• dragonkiez-buero.yml:ipv6_prefix: "2001:bf7:830:b300::/58" (dragonkiez: /56 subnets for all cores #1074) - flashed
• dragonkiez-dorfplatz.yml:ipv6_prefix: "2001:bf7:830:b340::/58" (dragonkiez: /56 subnets for all cores #1074) - offine, not flashed yet
• dragonkiez-rathausblock-miami.yml:ipv6_prefix: "2001:bf7:830:b380::/58" (dragonkiez: /56 subnets for all cores #1074) - flashed
• hds.yml:ipv6_prefix: "2001:bf7:810:b00::/57" (Cleanups - hds, radbahn, host_vars #1072) - nothing to do except merging the PR

[pmelange]

hds-containers is now online. It is running falter firmware. If it's wished, the router can be migrated to bbb-configs. But please make sure that "unifi" is resolved to 10.230.229.22 in /etc/config/dhcp