version up: zc.buildout, slapos.recipe.build, Debian 10/11 netinst

Thanks to the new buildout/slapos.recipe.build feature,
we can download the last released version of Debian Stable.

NEO stress testing is changed to run on both Debian 10 & 11.

Keep proposing Debian 10 netinst in kvm SR,
waiting that Debian 11 is more tested.