release-process: Policy for unmaintained branches.

release-process: Policy for unmaintained branches.

While only 2 branches are formally maintained (LTS and latest release),
OVS team usually provides stable releases for other branches too, at
least for branches between LTS and latest.

When transition period ends for an old LTS, we, according to
backporting-patches.rst, could stop backporting bug fixes to branches
older than new LTS.  While this might be OK for an upstream project
it doesn't sound like a user-friendly policy just because it means
that we're dropping support for branches released less than a year
ago.

Below addition to the release process might make the process a bit
smoother in terms that we will not drop support for not so old branches
even after the transition period, if committers will follow the
"as far as it goes" backporting policy.  And we will provide stable
releases for these branches for at least 2 years (these releases could
be less frequent than releases on LTS branches).

After 2 year period (4 releases) committers are still free to backport
fixes they think are needed on older branches, however we will likely
not provide actual releases on these branches, unless it's specially
requested and discussed.

Additionally, "4 releases" policy aligns with the DPDK LTS support
policy, i.e. we will be able to validate and release last OVS releases
with the last available DPDK LTS, e.g. OVS 2.11 last stable release
will likely be released with the 18.11 EOL release validated.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Kevin Traynor <ktraynor@redhat.com>