configitems: rename the config to prevent adding an alias in future
Right now the config option looks like:
[experimental.server]
stream-narrow-clones=
which does not match how config options are generally defined in core. So let's
rename this to:
[experimental]
server.stream-narrow-clones=
before the new release so that we don't have to add an alias in future for this.
Differential Revision: https://phab.mercurial-scm.org/D5198