Remove the requirement to authenticate for /admin/server_version. (#5122)

This endpoint isn't much use for its intended purpose if you first need to get
yourself an admin's auth token.

I've restricted it to the `/_synapse/admin` path to make it a bit easier to
lock down for those concerned about exposing this information. I don't imagine
anyone is using it in anger currently.

docs/admin_api/version_api.rst

@@ -10,8 +10,6 @@ The api is::
 
     GET /_synapse/admin/v1/server_version
 
-including an ``access_token`` of a server admin.
-
 It returns a JSON body like the following:
 
 .. code:: json