LU-13740 build: announce Ubuntu20 support

LU-13740 build: announce Ubuntu20 support

Now that Ubuntu20 is supported for both servers and clients lets
list it in the Changelog

Test-Parameters: trivial
Change-Id: Id413dc5bf5f8e418f379b8b9c8efc4e2d5521311
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/40088
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Peter Jones <pjones@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: Sebastien Buisson <sbuisson@ddn.com>