libgtk-4-1 Breaks/Replaces experimental libgtk-4-0

libgtk-4-1 Breaks/Replaces experimental libgtk-4-0

Both packages contain the print backends, which do not contain the
SONAME in their names. We anticipate that if the SONAME was bumped in
future, the "4.0.0" in the module loading path would also change, so
this isn't a problem for Policy §8.2; the only reason the path didn't
already change here is that versions prior to 4.0.0 were development
snapshots and explicitly not API- or ABI-stable.

Moving the print backends to a separate package would create a circular
dependency (since libgtk-4-1 would need at least a Recommends on the
print backends, and the print backends depend on the main library),
and would not actually help co-installability with a future libgtk-4-2,
since they should be upgraded in lockstep.