only warn about missing/invalid Date field for now

The Date field in the Release file is useful to avoid allowing an
attacker to 'downgrade' a user to earlier Release files (and hence to
older states of the archieve with open security bugs). It is also needed
to allow a user to define min/max values for the validation of a Release
file (with or without the Release file providing a Valid-Until field).

APT wasn't formally requiring this field before through and (agrueable
not binding and still incomplete) online documentation declares it
optional (until now), so we downgrade the error to a warning for now to
give repository creators a bit more time to adapt – the bigger ones
should have a Date field for years already, so the effected group should
be small in any case.

It should be noted that earlier apt versions had this as an error
already, but only showed it if a Valid-Until field was present (or the
user tried to used the configuration items for min/max valid-until).

Closes: 809329

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