os-release: define SUPPORT_END=

os-release: define SUPPORT_END=

Fixes #21764.

I think is very simple, but flexible. The date may be set early, for distros
that have a fixed schedule, but it doesn't have to. So for example Debian could
push out an update that sets a few months before the release goes EOL. And
various tools, in particular graphical desktops, can start nagging people to
upgrade a few weeks before the date.

As discussed in the bug, we don't need granularity higher than a day. And this
means that we can use a simple human- and machine-readable format.
I was considering other names, e.g. something with "EOL", but I think that
"SUPPORT_END" is better because it doesn't imply that the machine will somehow
stop working. This is supposed to be an advisory, nothing more.