Make OpenAFS 1.6.20.2

Make OpenAFS 1.6.20.2

Update configure version strings for 1.6.20.2. 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 1.6.20.2.
Switch to 1.6.21 dev 2 for macOS.

Change-Id: Iccc613ea6b7d1194e7a1b20fa38c54b192c3c7b4
Reviewed-on: https://gerrit.openafs.org/12532
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>