Remove backwards compat default DST for v2

In order to not break v1 users of the client, we pass the DST for beacon verification as an optional parameter and set some defaults.
This can be removed in v2 to ensure users don't accidentally try and verify a beacon with the wrong DST

Start by locating the beacon verification API and its optional DST parameter and defaults, then compare the intended v2 behavior with the current compatibility path. Done means v2 no longer silently supplies the backwards-compatible DST, so callers cannot accidentally verify a beacon with the wrong domain-separation tag.

Written by the indexing model from the issue text.