release-process: Standardize designation of new LTS releases.

release-process: Standardize designation of new LTS releases.

Standardize that we will mark a new release as LTS every two years
to avoid situation where we have a really old LTS branch that no-one
actually uses, but we have to support and provide releases for it.

This will also make release process more predictable, so users will
be able to rely on it and plan their upgrades accordingly.

As a bonus, 2 years support cycle kind of aligns with 2 years support
cycle of DPDK LTS releases.

Still keeping a window for us to discuss and avoid marking some
particular release as LTS in case of significant issues with it.

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