[yocto] RFC: dropping official support for Debian 8 / Opensuse 42.3

[Burton, Ross]

Hi,

Now that both Debian 8 and OpenSuse 42.3 are end-of-life and no longer
formally supported, we think it's time to drop them from the supported
distribution list.  Initially this involves removing them from the
SANITY_TESTED_DISTROS list in Poky, at some point during this cycle we
may remove those distributions from the Yocto Project autobuilder to
add more workers for other supported distributions.

It is expected that the next release will probably work on those two
distributions, there are no plans to do new and exciting things
dropping these unsupported distributions enables -- like increasing
the minimum Python version to 3.5 until after the 2.8 release in
October.

Please, if this impacts you, speak up now.

Ross