debian/{compat,control}: increase debhelper compatibility version to 12

Increase debhelper compatibility version to 12 as pointed out by
lintian:

P: tweeper source: package-uses-old-debhelper-compat-version 11

Also depend on debhelper-compat instead of debhelper and drop the
debian/compat file, as suggested by the following cme warnings:

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Warning in 'control source Build-Depends:0': compat parameter is deprecated. Please use debhelper-compat dependency. See debhelper(7) for details.
Offending value: 'debhelper (= 12)'
Warning in 'control source Build-Depends:0': debhelper dependency is deprecated. It should be a dependency for debhelper-compat package
Offending value: 'debhelper (= 12)'
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