Bump min supported release for building HEAD to 10.3

[emaste]

Summary

rS307825 and rS333017 disallow building FreeBSD-HEAD from 9.x or 10.x versions that have some clang issues.

Minimum supported SVN revisions and osreldates:

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 rS286035 fix. 10.3 is the first 10.x release with the fix.

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.

[emaste]

Also remove the special Clang warning in UPDATING and the entries prior to the stable/10 branch.

[imp]

Juniper specifically requested that we maintain this special case. They have (had?) a 9.x build farm that was up-to-date enough to build -current from. We should get confirmation that they no longer need it before we remove it.

The pruning of UPDATING (the second chunk) is a separate thing, and should be committed separately. I also don't think we should remove the UPDATING warning just yet.

UPDATING

35 ↗

(On Diff #41885)

This shouldn't be removed.

1428 ↗

(On Diff #41885)

This is a separate commit.

[emaste]

Committed without the UPDATING changes after @imp confirmed the special case is no longer needed by Juniper.

UPDATING update to be handled separately.