unbound: Update to 1.23.1

Release notes at
https://nlnetlabs.nl/news/2025/Jul/16/unbound-1.23.1-released/

Since we don't enable ECS, this is mostly a nop for us.

Merge commit 'c8864f6ba46ff3271d97b4ae1c3cc6ce01eaf18a'

MFC after: 3 days

(cherry picked from commit 5a0119491adbea0876d9d6dc0ef3e71f3d64a20b)