I just received an error message that v2 is no longer supported. I would have appreciated an email letting me know in good time that this was going to happen - I don't regularly login to Surveymonkey or Stackoverflow unless something has gone wrong.
Anyway, what is the quickest migration path?
Migration help doc is here.
It can be "bearer" or "Bearer".
Depending on when you created your app you may have an API key listed in the credentials section for your app. If you have an API key, then you can use it the same way you did in V2. If you don't have one, then follow the new authentication docs and you don't need to pass one in the URL every time. If you still have an API key, but would rather not have to include one for every call, then refresh your credentials and you'll no longer need one.
It doesn't matter if you put a trailing slash or not.
When in doubt, look at the examples in the V3 docs for formatting of content.