Make OpenAFS 1.6.20.1

Make OpenAFS 1.6.20.1

Update configure version strings for 1.6.20.1. Note that macos kext
can be of form XXXX.YY[.ZZ[(d|a|b|fc)NNN]] where d dev, a alpha,
b beta, f final candidate so we have no way to represent 1.6.20.1.
Switch to 1.6.21 dev 1 for macOs.

Change-Id: If9a54680d6807687136f6149ca48ad8c33db32f7
Reviewed-on: https://gerrit.openafs.org/12485
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>