main: rework launcher version checking

The code has an ad-hoc check in that it requires the launcher major
version to not be less than the source code version.  We don't really
care about that requirement, and it doesn't fit with our other version
checks.  Rework it so we explicitly declare the min launcher version
that is supported.

We'll start with requiring repo launcher 1.15 which was released back
in 2012.  Hopefully no one has anything older than that, although it's
not clear we work with even newer versions than that :).  But let's be
a little conservative with the first update to this logic.

Bug: https://crbug.com/gerrit/10418
Change-Id: I611d70c60324d313c76874e978b8499a491a5d00
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254278


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>