chg: dev: remove `sigrdataset` from `dns_view_findzonecut()`

Since the `sigrdataset` "output" parameter of `dns_view_findzonecut()` is never used (always called with NULL), it is now removed. As the resolver is moving towards a parent-centric direction, there is no point having a signature for the NS record (which is not authoritative in the parent, so never signed) when `dns_view_findzonecut()` is called.

Also, rename `dns_view_findzonecut()` as `dns_view_bestzonecut()` as it is used only in the context where the closest name servers for a name need to be queried and to avoid ambiguities with other code flows using `dns_db_findzonecut()`.

Merge branch 'colin/findzonecut-removesig' into 'main'

See merge request isc-projects/bind9!11444