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

Lustre-change: https://review.whamcloud.com/47550
Lustre-commit: ebb70f9b86fdf09e277cab0c91aecf5c4e16f5f2

Change-Id: Ie5c63f6a3eb29e9aeace1f008d8d492e7587adf2
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Thomas Stibor <thomas@stibor.net>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-on: https://review.whamcloud.com/47740
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>