Bump min supported release for building HEAD to 10.3

r307825 and r333017 disallow building FreeBSD-HEAD from 9.x or 10.x
versions that have some clang issues. The minimum supported Subversion
revisions and osreldates containing the fix are:

Version Min Rev osreldate

 9.x  r286035     903509
10.x  r286033    1002501

9.3 is the final 9.x release and does not contain the r286035 fix.
10.3 is the first 10.x release with the fix. Thus, in practice 10.3 is
the oldest release that can build HEAD.

Although it may still be possible to build HEAD from an up-to-date
stable/9 it's not worth maintaining the special case when the branch
itself is unsupported and there are no usable releases from that branch.

Old UPDATING entries can be removed and the Clang warning in UPDATING
may be updated, in a future commit.

Approved by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15209