Cherry-pick ICU-20975 BRS66RC Update version numbers, README files, configure

Cherry-pick upstream commit
https://github.com/unicode-org/icu/commit/925f2550924f9b275d21ae801bc8b52f3bd0aa18

Test: n/a
Change-Id: Idaa0dd15013e4d5d5f4d0fc582fdf691bee08d91
13 files changed