Bump severity of FTBFS against gnutls 3.4 bugs

[Andreas Metzler]

# GnuTLS  3.4 has been declared stable and I intend to upload to sid
# soonish. I have already opened a transition bug (#810597) and am
# simply waiting for a slot. I am therefore raising the severity of
# the few remaining bugs regarding FTBFS against gnutls 3.4.

severity 624025 serious
severity 624074 serious
severity 782801 serious
severity 782828 serious
thanks