FAQ: update eol dates with rough estimate

FAQ: update eol dates with rough estimate

Currently we do not yet have fixed dates for Stretch, but we can
estimate it to be about a year after Buster release on 06.07.2019, so
~ XY.07.2020, probably. Try to be rather strict, as it's easier to
postpone this date than to prepone this date. Users won't be angry if
they get support a month or two longer, but probably are if it's cut
for a month or two,

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