release-process: Add transition period for LTS releases.

release-process: Add transition period for LTS releases.

While LTS change happens, according to release-process.rst, we're
immediately dropping support for the old LTS and, 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
(some upstream projects like QEMU doesn't support anything at all
except the last release) it doesn't sound like a user-friendly policy.

Below addition to the release process might make the process a bit
smoother in terms that we will continue support of branches a little
bit longer even after changing current LTS, i.e. providing at least a
minimal transition period (1 release frame) for users of old LTS.

Effectively, this change means that we will support branch-2.5 until
2.15 release, i.e. we will provide the last release, if any, on
branch-2.5 somewhere around Feb 2021. (I don't actually expect many
fixes there)

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