bird2: update to 2.18

bird2: update to 2.18

Version 2.18 (2025-12-26)
- BGP: Support for dynamic onlink and link-local connections
- BGP: Listening socket refactoring
- BGP: Fix restart behavior on outgoing next hop setting
- BGP: Configuring global/link-local IPv6 nexthop
- BGP: Disallow AS Sets by default
- L3VPN: Support for import/export target none and import target all
- RAdv: P-flag to prefer prefix delegation in DHCPv6
- Filter: Merging of case intervals
- Filter: Append operator
- ASPA: Paths containing AS_SET are invalid
- Doc: Update website to bird.nic.cz
- CI: Upstream packaging cleanup and reproducible builds
- Various minor fixes in code, comments, documentation and tooling

Signed-off-by: Packet Please <pktpls@systemli.org>