Update Build-Depends to use debhelper (>= 9.20160709) instead of dh-s…

[ayourk]

…ystemd.

Update Build-Depends to use debhelper (>= 9.20160709) instead of dh-systemd or key it on a newer version of debhelper if you wish.

[the-ress]

I have a similar change in my PR, but I just removed dh-systemd and kept the original debhelper (>= 9).

Did you use the >= 9.20160709 requirement specifically because that's the first version with dh-systemd merged in it, and any previous version wouldn't work?

[ayourk]

I based this upon another package with this specific number or possibly some Stack Overflow article. I'm not certain when dh-systemd was merged in.

[the-ress]

Thanks. It looks like that version is indeed when dh-systemd was merged in: matrix-org/synapse#9073

I updated my PR (#346) to require that version too.