Bump debian package compat level to 9

Summary:
Compat level 9 is recommended by Debian: https://manpages.debian.org/jessie/debhelper/debhelper.7.en.html#COMPATIBILITY_LEVELS
Also, compat leves 7 and 8 are deprecated, despite that doc not indicating such.

Test Plan: Used to run this build successfully: https://launchpad.net/~jasonbcox/+archive/ubuntu/bitcoin-abc/+build/18829037

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5467