d/control: define compat level via build-depends and raise to 12

d/control: define compat level via build-depends and raise to 12

we might upload this 1:1 to bullseye, so leave at level 12 for now to
avoid using debhelper from backports or downgrading this there again,
if one ever wants to build it on bullseye directly.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>