LU-15918 build: update debhelper compatibility level to 11

LU-15918 build: update debhelper compatibility level to 11

This patch updates debhelper compatibility level to 11 (which is
currently the recommended level) to resolve the following build
warnings while running "make debs":

Compatibility levels before 10 are deprecated (level 9 in use)

Test-Parameters: trivial clientdistro=ubuntu2004 \
testlist=sanity

Change-Id: Ie5c63f6a3eb29e9aeace1f008d8d492e7587adf2
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/47551
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>