cleanup: api-microversions spec

The spec was using some unsupported formatting:

* A list
** This is not how to indent to the next level
    * This is

Also, dropped the link to semver.org from the references as
we have decided to pursue an approach which does not use
that.

Change-Id: I777cc98a10d82c0e3e28b8324aeaf56e7ff4c160